Many others have attempted to define the roles within user experience, but there are no hard boundaries.
Jesse James Garret descibes in his book The elements of User Experience (See overview) includes Usability as part of the Strategy Plane, although I'm sure he would agree that as in so many situations, the components of user experience do not fit neatly into little standalone boxes, but spill out into other areas. [Note that JJG adapted his top level 'element' from Visual Design to Sensory Design]
So, I think the Bible (That's Information Architecture for the World Wide Web by Peter Morville and Louis Rosenfield) very nicely describes various roles within the industry:
I quote directly from Chapter1 of the book: (which I heartily recommend in order to find out about the various methodologies you ask for, as I feel that is too wide a topic to answer here)
Gray areas and caveats aside, here is our attempt to draw some
boundaries between information architecture and a number of closely
Traditionally, a graphic designer was responsible for all aspects of
visual communication, from the design of corporate logos and
identities to the layout of individual pages. On the Web, we’re seeing
increasing specialization due to the complexity of the environment.
Even so, many graphic designers do a great deal of information
architecture as part of their work.
Interaction designers are concerned with the behavior of tasks and
processes that users encounter in software and information systems at
the interface level. They often have a background in human-computer
interaction, and are focused on helping users successfully achieve
goals and complete tasks.
Usability engineers understand how to apply the rigors of the
scientific method to user research, testing, and analysis. Their
background in human-computer interaction and their experience
observing users provide them with useful insights into design. They
are often concerned with testing all aspects of the user experience,
inclusive of information architecture and graphic design.
Experience design is an umbrella term that encompasses information
architecture, usability engineering, graphic design, and interaction
design as components of the holistic user experience. You’ll find
relatively few “experience designers,” as there aren’t many people on
the planet with skills in all these areas. The term is useful insofar
as it encourages cross-disciplinary awareness and collaboration.
People rarely confuse software development and information
architecture, but the two fields are highly interdependent.
Information architects rely on developers to bring our ideas to
fruition. Developers help us understand what is and isn’t possible.
And as the Web continues to blur the distinction between software
applications and information systems, these collaborations will become
even more important.
Content management and information architecture are really two sides
of the same coin. IA portrays a “snapshot” or spatial view of an
information system, while CM describes a temporal view by showing how
information should flow into, around, and out of that same system over
time. Content managers deal with issues of content ownership and the
integration of policies, processes, and technologies to support a
dynamic publishing environment.
Knowledge managers develop tools, policies, and incentives to
encourage people to share what they know. Creating a collaborative
knowledge environment means tackling tough issues surrounding
corporate culture such as “information hoarding” and
“not-invented-here syndrome.” Information architects focus more on
making accessible what has already been captured.