A whole note looks like a small hollow oval that is slightly wider than it is tall. The composer intends for you to play or sing any whole note for a fairly long amount of time. How long can be determined by tempo indications at the start of the composition or by personal taste.
A half note looks like a whole note with a vertical line either going down from its left side or up from its right side. The oval may also be rotated slightly in a counterclockwise direction. Play or sing a half note for half the amount of time as a whole note, which may still be somewhat long.
A quarter note looks like a half note with the oval filled in so that it is solid black. Play or sing it for half the time as a half note, which is one-fourth the amount of time as a whole note. Quarter notes are perhaps the most common type of musical note.
An eighth note looks like a quarter note with an additional line coming off the end of its vertical line. This line might connect it to one or more eighth notes. Play or sing an eighth note for one-eighth the amount of time as a whole note.
A sixteenth note has two parallel lines coming off the end of its vertical line. Play or sing it for one-sixteenth the amount of time as a whole note. It is possible, but rare, to continue subdividing the note by adding a third parallel line to make a thirty-second note, a fourth to make a forty-sixth note, etc.