I was browsing through the web and I came across this sentiment:
Dear Skype, this (red X) button means close, not minimize
I also found the same question on the Skype website. Why would a company like Skype do something that is so different from the standard? Microsoft Lync (office communicator) also has this design of using X to minimize.
What user experience reasons could cause such large companies as Microsoft and Skype to choose a non-standard behavior for the X button?
I believe this application (Skype) and many other communication type applications including instant-messengers, email clients and other VOIP apps, hi-jack the "X" button to minimize the more user-frustrating event of accidentally ending a users communication session. In many cases, users might simply want to get the application of the screen, the fastest and most common way is to simply click the "X".
By leaving the applications running in the background users still appear to be in an online/available status.
Skype, being a peer-to-peer telecoms application, works much like BitTorrent and other P2P distribution methods by relying on users' own machines and internet connections to route the traffic of other people's calls. This means that, as a Skype user, your machine is being used to facilitate other people's connections even when you aren't making a call yourself, much as a BitTorrent user is serving data to other users when he or she isn't making a download themselves.
As such, it's not in Skype's interest to allow users to easily close the application, as that means that more traffic has to be handle by Skype's own servers, which naturally comes at a cost. Most Windows users don't have visibility of any applications not on the taskbar, so by hijacking the close button in this fashion, Skype can ensure that there's always peer-to-peer nodes available to handle new calls.
This is, of course, highly cynical, and breaking native OS convention in this fashion is extremely bad practice. So don't do it.
In OSX it is very common for closing a window to not remove the application from the dock. In many applications the window represents a document in the application, while the application itself doesn't have a window. There are also applications like iTunes and Spotify, that don't require a window to continue to play music. Skype doesn't need to have a window for you to be available for a session.
Applications that are still running but don't have a window, are available in the dock. This way you can select the application and open an new window if you need to create a new document or interact with the application in any way.
In Windows it has always been the rule that an icon in the task bar is related to a window. Minimizing a window temporarily removes the window from the view, but it's still "there". If the window represents a document, closing it will close the document and if it's the last window of the application it will also quit the application. If the window represents the application itself closing it will quit the application.
Applications that don't really require a window have always been looking for ways to properly dealing with this. Some, like media players, "minimize" to the system tray. Chat programs like Live Messenger have always had this feature of minimizing to the tray. This puts these applications out of the way of applications on the taskbar that you do interact with actively.
In Windows version before 7, application windows used to take up a lot of space on the taskbar. This is quite different from OSX where an application could have any number of windows only represented by a single icon in the dock. Having items on the taskbar for applications that are running in the background, to keep your music going or keep you "online", would be very unpractical if the taskbar also needs to give access to a mail client, some Word documents, etc.
In version 7 Windows switched to having all windows of an application represented by a single icon, just like OSX. This opens up the possibility of having a media player or chat client active in the task bar. Furthermore, the default behavior of the system tray has gone to hiding all icons. Therefore it's no longer practical to minimize applications to the tray, as the icon will be hidden and users may be unaware the application is still running. So along with the new design of the task bar, applications are discouraged from using the tray if they're running in the background.
These are all steps on the Windows platform towards a window management paradigm that is closer to that of OSX. It no longer really makes sense that closing the window of an application should quit the application if the application could do a lot of useful things without a window. We're moving towards a Windows environment where the window is the application itself but merely a document or a view of that application. It is a recognition of the fact that the windows paradigm is becoming outdated.
At this point this "halfway there" situation causes confusion. Applications that have grasped the opportunity to move ahead towards this paradigm work differently. For one, minimizing the window (removing it from view) accomplishes the same things as closing the window. This is not all that different from OSX's behavior. There, minimizing a window moves it to a separate section of the dock. However, for applications like Skype and Spotify, this accomplishes the same thing as closing the window (except that it takes up space in the dock).
I think we're witnessing a slow movement towards figuring out how a WIMP environment should work. We've been sort of fine since Windows 95, but it's far from ideal and OSX and Windows 7 offer many improvements for usability and that's only the beginning. It will be a long and bumpy ride though, because some applications are built in a way that makes them very hard to move to a new windowing paradigm. Just look at Microsoft's inability to solve problems in this area for Excel. They've moved Word to be smarter about what a window is and does, but Excel is still stuck in the 90's. On the other hand fresh applications will be trying out new possibilities and moving the ecosystem forward.
A lot of application designers - and I am one - suffer from the delusion that their application is the most important application that a user has the privilege of running on their system. They simply cannot imagine that a user would not want keep their application running. Or auto starting for that matter. So, they come up with tricks like minimizing on clicking the X, popups asking if you're sure you want to quit, make the application auto-start, etc. Skype is no exception unfortunately.
However, why specifically Skype is doing this is always going to be speculative, unless you manage to attract the attention of the designers of Skype to your question and get them to answer. So, there is no definitive answer possible.
Skype is not the only application which does that. This is a standard behavior of Mac OS applications too.
When you select the 'x' on the window, you are closing the window. Closing windows has different meaning in different applications, in chrome, you are effectively closing all your tabs, but in itunes or outlook, you are effectively just minimizing the window.
The application is still running. To stop the application you need to explicitly quit.
Mac Basics: Closing windows versus closing programs
Unlike in Windows, when you press the red button on a Mac's window it closes the window only, and generally does not close the parent application for that window. In a few applications like Calculator, closing the window will close the application itself, but usually this is not the case. In the case of Safari and most other applications, when you click the red button the window closes but the Safari icon in the Dock has a small white dot under it that means the program is still open.
This is the explanation for the interaction, but, why skype does this is not something I clearly understand, since skype is MS and none of the other MS applications on windows exhibit this behavior (as far as I remember).
I can speculate that since skype is a service which can run in the background like an antivirus, quitting the window should not quit the process, rather just close the window. But, even then skype icon hogs up the space in your application bar.
Edit:
Windows doesn't require applications to quit on the window being quit.
Window Management Guideline for windows 7:
Close. All primary and secondary windows with a standard window frame should have a Close button on the title bar. Clicking Close has the effect of canceling or closing the window.
They have a tiny exception in case of running processes:
Exception: For progress dialogs, you may disable the Close button if the task must run to completion to achieve a valid state or prevent data loss.
But, nothing proving that skype interaction is wrong.
OS X Human Interface Guidelines:
Under Window Components:
Every document and app window and panel has, at a minimum:
... A close button, so that users have a consistent way to dismiss the window.
Skype (and other communication applications) need to keep running in the background in order to be able to receive messages and calls.
For communication applications (and a few other application categories, antivirus for example), this is - as far as I know - pretty much standard behavior.
-like with most programs.