The Complete Idiot''s Guide to Music Theory

Part 2:
Rhythms
the four-beat duration of a whole note. Put another way, you can put two half
notes in a measure, because two half notes equal one whole note.
A half note looks like a whole note with a line next to it. The line is called a
stem;
it can point up or down, depending on the pitch of the note. If the stem
points down, it sits to the left of the notehead. If the stem points up, it sits to
the right of the notehead.
60
Two half notes—one with the stem pointing up; the other with the stem pointing down.
If the 
notehead
(the part of the note that isn’t the stem) is on or above
the third (middle) line of the staff, then the stem should point down from
the notehead. If the notehead is below the third line of the staff, then the
stem should point up from the notehead. 
The only exception to this guideline is when you have a run of connected
notes—notes where all the stems are connected, like a run of eighth or sixteenth
notes. It’s okay to run two or more of these connected notes together with their
stems in the same direction, even if they move past that third line of the staff, to
make the music easier to read.
Tip
When you play a half note, make sure the tone lasts a full two beats. If you let
up after the first beat, you’re playing only half a half note—which is what we’ll
discuss next.
Quarter Notes
Let’s keep going. If a half note is half a whole note, what is half a half note?
Well, do your math, and when you divide 
1
⁄
2
by 2, you get 
1
⁄
4
. This means half a
half note is a 
quarter note.
Because a half note lasts two beats, a quarter note—which is half that duration—
lasts one beat. Put another way, you can fit four quarter notes in a measure; one
to a beat.
When you tap your foot to the beat of most popular songs, your foot is tapping
quarter notes. One, two, three, four—each of those counts is a quarter note.
A quarter note looks like a half note with the 
notehead 
filled in, as shown here:
A
notehead
is the
big, oval part of a note.
The stem is always
attached to the notehead.