LARP stands for Live Action Role Play and is the dress up in costume and act it out variety of RPG which stands for Role Play Game.
these are (usually) fantasy or sci-fi adventure games which can be played online, with a set of cards and dice or similar (often reffered to as table-topping) or acted out in real time by people (LARP).
In all varieties you take on a character, there are various skill sets or attributes, and other thingimajigies which you can use and sort of fill out the rules of the game.
all very good fun and very odd at times.
(this is the game i play most frequently, if you want to get a better description than my ramblings)