Eighth note triplets are used the most since they divide equally across one beat. An eighth note has a stem coming off of a solid round circular note and a single flag on the end of the stem. An eighth note triplet occurs in groups of three with a beam and the number three placed over the top of the bracket. The three indicates that it is a triplet.
Sixteenth note triplets are significantly faster than eighth note triplets. The 16th note is exactly twice as fast as an eighth note and looks identical, except for an extra flag that shoots off the stem. The 16th note triplets started appearing heavily in music in the 20th century. These triplets have the same method of notation that an eighth note triplet does. Three 16th notes appear under a beam with the number three on top. In one beat, you can fit six 16th note triplets.
Quarter note triplets are usually found more often than 16th note triplets, but quarter note triplets do pose some performance problems. A quarter note triplet uses a bracket instead of a beam placed over three quarter notes. Quarter note triplets are more difficult to play since they span the length of two beats. Often, performers will subdivide by thinking of 6 eighth note triplets and just playing every other triplet. The quarter note has a stem and a single black circular note on the end.
Half note triplets rarely occur as they span the space of four beats. A half note looks like a quarter note, except the circular portion appears hollow. To count a half note triplet, most performers will use the same subdivision technique used for quarter notes. Instead of 6 eighth note triplets, they will think in terms of 12 eighth note triplets and play every fourth triplet to receive the three half note triplet feel.