The_Complete_Idiot%27s_Guide_To_Music_Theory

The most common rhythmic notes—each smaller note is exactly half the previous note.
Whole Notes
The most basic note is called the whole note, which is called a whole note
because, in 4/4 time (we’ll get to this soon—I promise!), it lasts a whole measure.
Because 4/4 time has four beats in a measure, this means a whole note lasts
four beats. A whole note looks like a big empty oval, nice and whole.
Whole notes, at different places on the staff.
When you put a whole note in a measure of 4/4 time, that’s the only note that
goes in that measure; no other notes will fit. When you play a whole note, you
play your instrument once at the very beginning of the measure (on beat one),
and then you hold that tone through the entire measure. In other words, one
whole note should last a whole measure.
Half Notes
Because the whole note is the largest note, let’s go down one size and look at the
next smallest note. Remember that I said each shorter note is exactly half the
previous note? Let’s apply a little math and divide a whole note by two to see
what we get.
The next smallest note is called a half note. (It’s half of a whole note—pretty
simple, eh?) Because a whole note lasts a whole measure (in 4/4, anyway), a half
note lasts a half measure. This means a half note lasts two beats, which is half of
Chapter 5: Note Values and Basic Notation
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