SET? What?

Ok, I have been pouring over stuff in my PostgreSQL book and the online
docs and stuff.. but I can't nail down what the hell a "SET" is. I hear
you guys use "tuples" a lot and I know SET defines somekind of type with
tuples, but what ARE They and how would one set them?

I kinda assumed ( probably wrong ) they were like either the SET or ENUM
of MySQL, but now I am just lost. What ARE these tuples?

best regards,
dan