OK, a brief run-down:
A LARP is a Live Action Roleplaying Game - these are realspace events with a number of people, who each act in the role of a single character. LARPs often use more or less elaborate costumes and props, and place a high emphasis on being "in character". They will often include a mix of discussion and physical/combat elements, with specific safe combat rules in place and referees to ensure fair play.
A Megagame is similarly a realspace or mostly realspace event, and similarly puts people in specific roles. However, a megagame is more likely to be a one-day event and have a specific scenario, rather than being an ongoing campaign; they may also include more computerised elements, and the players are more likely to be "control figures" negotiating and planning with one another rather than front-line fighters. Megagames rarely include direct physical and combat elements, and will often place more emphasis on your character's function (e.g. "You are the ambassador of Jubalopolis, you have these functions and aims") rather than roleplaying a particular personality.
