LARPing is as varied as any other form of roleplaying.
At its most basic, its esentialy roleplaying using yourself as the miniatures.
Some forms are very combat oriented... Being good with a foam sword is quite important in these type, as well as the ability to make said swords, and armour and such. Bruises happen. These tend to be the most fun, IMO. Spellcasting is often handled through things like throwing beanbags for fireballs, etc, or it's just arbitrated by a game master. This type of thing is normaly done in fields, woods, etc, and may or may not include multi-session storylines.
Another type of the sterotypical whitewolf LARPer... these tend to be political socio-dramas, if you ask them what they are... Personaly, I don't see much behind people with white makeup pretending to such blood while kissing up to a prince character for favor.
Some are more official than others, there are several nation-wide (and international) larping groups out there... IFGS is good for DnD type larping. These are good because they tend to have standardized rules, so they don't turn in to a "No, that counts! No, it doesn't!" arguement, as well as an enforced level of saftey. I use the IFGS rules, though I'm not a member of IFGS. It includes rules for how to set up locks for thieves to pick, which is really cool

... but beyond that, it's just some fairly decent larp rules, even if it errs on the side of caution as most "official" larps do.