A slight amplification of Little Robyn's list:

C to c = Ionian
from G to g = Mixolydian
from D to d = Dorian
from A to a = Aolian

Ionian is your everyday major scale.
Aeolian is minor.
Mixolydian is "Old Joe Clark" and "She Moved Through the Fair".
Dorian is "Scarborough Fair" and "What Do We Do With a Drunken Sailor?".

Obviously, no one needs to know the name of the mode or how it is constructed in order to play music, either by ear or by reading. Where some practical knowledge of modes becomes useful is if you're finding the guitar chords for a song, coming up with a harmony, doing any kind of arranging.