Meetups (and specifically, Maker Meetups) tend to be organized public events where anyone and everyone who is interested in the topic (Making, in this case) is invited to take part. Meetups can be formal, or informal, and may include talks or presentations, or may lean towards being more social. Meetup topics run the gamut, and many groups who are trying to start a makerspace start the process with meetings or meetups.
If you are a member of a makerspace your space may host meetings for internal groups within the space, or for outside groups who hsve interests that cross over into the maker world.
If you attend Maker Faire or a Maker Fest you may hear about events like Bring-a-Hack, which are meetups that happen during large events and are meetups for makers/hackers to socialize and share neat stuff they are making.