What do "M", "G" and "B" buttons mean in an elevator, besides "1", "2", ...? Confusing. Thanks.
"G" is "Ground Floor". In some countries, the bottommost floor of a building is the ground floor, and the floor above that is the first floor. In the US, ground floor and first floor are used interchangeably, with the next floor up being the second floor.
"M" is "Mezzanine". This is sort of a "half floor" that doesn't extend across the entire span of the building and is therefore not numbered on its own.
"B" is "Basement", the floor below the ground. In buildings with multiple underground levels, the basements are numbered going downward, so the level closest to the ground level is the first basement "1B", the one below that is "2B", and so on.
As @choster points out in the comments, these aren't the only possibilities, depending on the location (metro station, parking garage, etc), but I believe they are the most typical meanings in most standard buildings.