Recognition
The fraternity of Freemasons developed in a very interesting decentralized fashion. Lodges formed independently of one another and gave rise to Grand Lodges to govern them but no authority would ever govern them all. In the first century of the modern Grand Lodges there were often many grand lodges competing for control of a jurisdiction. Most often however there was a dominant grand lodge that would be undisputed by the vast majority of local members. One of the largest schisms was the fifty year rivalry between the Antients and the Moderns in 18th century England.
Since the fraternities inception there has been a very simple mechanism to separate out authorized and unauthorized lodges. It was called "recognition" and it simply meant that one grand lodge would officially recognize another and visa versa. This status of mutual recognition is what makes it allowable for members of those jurisdictions to visit and interact with one another in an official masonic capacity.
To this day there continues to be no shortage of unrecognized lodges and grand lodges all around the world. As a general rule, if a grand lodge of recognized by The United Grand Lodge of England then it is considered to be within the sphere of "mainstream masonry". From time to time individual grand lodges may extend or revoke recognition of other grand lodges for a number of reasons. A high profile modern example of this is that of the UGLE discontinuing recognition of most of the lodges in France.
If you are not a mason and desire to become one, it is well worth your time to investigate the proper Grand Lodge in your area and make sure it is part of mainstream masonry and not an unrecognized imitator.
It is likely that at some point in your masonic career you will encounter someone who says they are a mason but not a member of a recognized jurisdiction. Many times an individual will not even realize that their grand lodge is not legitimate. These individuals are no different than anyone else you meet in the world and should be treated with the same respect and courtesies you afford to every man. Not being a member of a recognized grand lodge simply means they cannot attend your tiled meetings nor you theirs..