agile (with a little a) is about delivering value to your customers as quickly as possible. It does this through constant collaboration, feedback and iteration. Originally it was conceved as a reaction to the slow moving, documentation heavy "Waterfall" projects of the 1990's.
While the term agile has been around since 2001 it isn't static and opinions have grown from the initital manefesto. If you're interested in more recent takes on agile have a look at Modern Agile and The heart of agile.home