In guts players pay an ante and receive two cards. Then players must all declare at the count of three whether they intend to contest the pot (e.g. you drop a chip on the table in front of you if you're in). If you lose, you pay the amount in the pot. Flushes and straights do not count.
In three card guts, flushs and straights do count. A variation is to have a one card draw after declaration.
A variation for declaration is to have players declare in order around the table.