Larp is an acronym from "Live Action Role Playing" that the role playing community has started to use as a term.
Larp is a atmosphere-, battle- or adventure game with a specific topic or theme. Basically the organizers (us), organize for players (you) an event, similar to a festival or a theme party.
The players choose or write their own character according to the topic of the event and send those character descriptions to organizers, together with their registration info. After that the players find/rent/make themselves costumes and game equipment or props (again according to the theme and their character). Meanwhile the organizers look through those character descriptions and try to incorporate them into the theme or story they have in mind.
Current larp is in a pseudo medieval setting. Players use real costumes (even metal armor) but padded imitations of real weapons (toys). Therefore anything medieval or ancient is in its place but also elements from fantasy or fiction, literature and movies. You can read more about the background of the game here. You can find out more about how to create your character for this game, here.
During the game the players will try to fulfill different tasks, set before them by the organizers (Game Masters). They can act individually or group together, depending on the task and/or their choice. The difficulty of the tasks varies and the exact solution to each problem is up for the players to decide. This is a social entertainment where you must interact with others to succeed.
The use of weapon imitations and to a limited degree also player interaction is governed by a specific set of rules, which can be obtained here. This larp involves a lot of mock combat. Fighting is not mandatory but a lot of the tasks given by Game Masters involve it.
This larp is organized as a non-profit event. The Game Masters and organizers are volunteers who don't get paid for this. All expenses involved with running an event of this scale are covered from donations and the participation fees that players pay. If there is any excess money it goes towards improving props and making the game atmosphere even better from year to year.