A whole note (also called a semibreve in British English) is the longest note typically found in music notation. It is notated with only an open note head (like a half note), without a stem.

Here it is on a staff:

A whole note has a duration equal to two half notes:

… or quarter notes:

It’s critical not to confuse notes and beats. The value of a whole note is simply a whole note… or two half notes, or four quarter notes – a whole note does not equal four beats as is very often taught! Only in common time (4/4), in which there are four quarter note beats per measure, does a whole note equal four beats.

(For more advanced readers: A whole note is the only rhythmic unit whose length may be undefined. It is sometimes used to indicate an entire measure, but only for measures totaling at least four quarter notes. For example, in an irregular meter such as 5/4, a whole note may be used to denote all five beats in a measure.)

If you wish to master reading music, be sure to check out my comprehensive course, How to Read Sheet Music.