That's a difficult question, since of course only a Freemason would be able to give you a full answer, and they aren't allowed to tell you! They are a men-only organization, although there are special groups for women; they have certain secret rituals; they are supposed to help each other and, although this is sometimes denied, support each other in business and life; and they do charitable work.

I have known masons, and to an outsider it seems that mostly they just like to meet, do some networking, meet their friends etc, rather similar to rotarians and other such organizations. However, there is a lot of curiosity about what they get up to in their rituals, and books have been written that claim to expose them. Some of these are from a religious perspective, saying that masonry is anti-religion, but this seems unlikely as many masons are churchgoers. Others claim that they are behind all sorts of conspiracies, but like all conspiracy theories this one is very short on evidence.