ENUM is for holding one among many choices. For example, suppose you're asking for someone's favorite color, and you want to limit the choices to just six 6: red, blue, green, yellow, orange, and purple.
Of course, you can't have two favorite colors, so the best option is to store their reply in an
ENUM('red', 'blue', 'green', yellow', 'orange', 'purple').
Why do this instead of VARCHAR(6) or TINYTEXT?
1) It takes less space - the VARCHAR option will take up at least 6 bytes per row, even if it's NULL, and TEXT will take x + 1 bytes, where x is the number of single-byte characters, (the 1 byte is used to store the length of the string). ENUM, however, since it only has to record one choice, only needs 1 byte for up to 256 choices, and 2bytes for 256 to 64k choices!
2) Searching ENUM fields is faster.
3) In case they didn't submit a value, or to be flexible (because you can't possibly list ALL the favorite colors in the world), and also to make your PHP life easier, you can set a default value with ENUM:
ENUM('red', 'blue', 'green', yellow', 'orange', 'purple', 'other') NOT NULL DEFAULT 'other'
Does that help?
p.s. Maybe you've noticed that ENUM is a lot like a radio button. There's a checkbox version, too: SET
