Not exactly. You don't have to dress up. It is basically RP'ing, but... Well, live action. In my game we don't dress up or anything. We just make the necessary character sheets, background story, etc.
We just act how our characters would.
Which is awesome. I give my players bonus EXP if they role play their characters (yes, I'm a DM too).
Larping is going out, and physically putting yourself in the position of a made up character in a made up setting. Outside, in a field or a forest or whatever, generally with a ton of other people. There's no dice, no character sheets, no levels. If you want to go out into a field and play D&D, that's totally cool, but that doesn't make it LARP. The only similarity between the two is that role playing is involved, but they are two widely different activities.
It is basically RP'ing, but... Well, live action.
I hate arguing semantics, but it isn't "basically" role playing, it
is role playing. LARP literally is the embodiment of role playing. You are an actor, playing the role of someone else in a fictional setting.
Taken from the Wikipedia page:
Players physically portray characters in a fictional setting, improvising their characters' speech and movements somewhat like actors in improvisational theatre.[15] This is distinct from tabletop role-playing games, where character actions are described verbally.
I'm just saying.
/end nerd rant
