I've played many and various larps over the past... 15 years or so.
Larps can be roughly divided into two different camps - "live combat" and "theatre style". In live combat games, combat is at least in part simulated with simulated weapons - folks run around whacking each other with boffer weapons, or shooting each other with paintball/airsoft weapons. In theatre style games, combat is simulated with something more like a tabletop rules.
So, Mind's Eye Theatre (the World of Darkness larp rules) are theatre style. NERO and Alliance are live combat games.
Some larps are stand-alone, weekend-long, day-long, or a few hours games, others are multi-year chronicles. Some use very light rules (or almost no rules at all), others have very full rulesets (like "Mind's Eye Theatre" or "Rules to Live By"). Some are run by large organizations (NERO, the Camarilla), and others are run by a couple of folks who get together to write. In some you get to create your own character, in others characters are pre-generated and you get cast into a role...
I tend to more theatre-style stuff, though the chronicle I'm currently in has a mix of forms, and a full ruleset that they sometimes work by, and sometimes don't
Those on the east coast of the US who are curious might want to look into one of the "Intercon" larp conventions. One runs in Boston every year, and another down in the Baltimore-Washington area. Intercon runs a whole bunch of minigames, in a variety of genres and styles, so you can get a bit of a taste.
So, rather like tabletop games - experiences can be all over the place.