In the case of web applications, "context" is where user and environment meet, and the many variables that will impact the user experience are:
For the User:
- What they are doing at the time (Working, relaxing, shopping,
responding to an emergency)
- How much stress they are under (Minimal,
low, moderate, high)
- Where they are (indoors on the couch, outdoors,
on a boat or a bus, at an office, work, home, hospital, breakdown
lane of the freeway)
- Who are they with? Are they being interrupted,
or are they able to focus? Is it quiet or noisy?
For the System:
- Is it fighting with other systems or apps for resources?
- What is the application being used to do?
- How much screen real estate is this app allowed to take up?
- Are there other systems the user must access in concert with this one, to perform their task?
I know I have made the mistake of assuming my application is the only one the user is running, or paying attention to, when that may not be the case at all. It is a wake-up call to visit customers in their offices or homes, or on the road, and see how our applications must co-exist with other tools they use, and how that impacts their perception of the app, and their usage of it.
For example, say I am far from home, driving at night, and I have a catastrophic steering failure. I pull over. My hands or shaking, and I cannot type. It's so much easier to push and hold a button, and say "tow trucks near me" than to fire open the map, and search for towing services in an unfamiliar city. Context, in this case, is everything, as I stand in the breakdown lane at night, clutching my iPhone and realizing this is the very first time I have ever used Siri. And it works well, allowing me to handle this situation despite my agitated state.
Technically, it's a similar task to searching for an art supply store on my iPad, from the couch, but since I'm not having an "art emergency", the context of my task is different on several dimensions, and my typing skills are not compromised. I can work this task very differently and still have a satisfactory result.