In music notation, a half note represents two beats and is represented by an open circle with a stem. It is so named because it lasts for the duration of half of a whole note. Also called a minim.
A series of half notes on the staff looks something like this:
/\
---| /-----------------------------------------------------
|/ |
---/------------------O------------|-----------------------
/| O | O |
-/-|/\---------|-----|-----|-------|-----------------------
| | | | | | O
|--|--|--------|-----|-----|-------------------------------
\ | | | |
--\|_/-----------------------------------------------------
|
\|