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The concept of "Contexts" is taken from David Allen's "Getting Things Done" methodology and refers to a setting in which an event takes place. When you tackle a task it should be based on the context that you're in. For example, there's no point trying to make an important phone call when you're in a library but you could write a letter your aunt thanking her for your birthday present.
You can find much more info on the web by searching for "gtd context". One example is here: http://www.43folders.com/2006/07/31/simplify-contexts