GID, current, primary, supplementary, effective and real group IDs?

The following links discuss these concepts in different contexts. I have read their definitions, but I still can't tell how they are related, or if some of them are just the same. * Current group ID * Group ID * Primary and supplementary group IDs * Effective and real group IDs (also on Wikipedia ) ### Here is one example of the source of my confusion: According to man id , if I type id, I should get what they call **effective** and **real** group IDs. id uid=501(joe) gid=501(joe) groups=501(joe), 100(users) However, Wikipedia refers to the output of id to distinguish between **primary** and **supplementary** IDs. Moreover, Wikipedia distinguishes between **primary** vs **supplementary** and **effective** vs **real** group ids. How do these concepts relate to each other? Also, is it true that **primary** group ID = **group ID** = **current** group ID?