In music notation, a quarter note represents one beat and is represented by a filled circle with a stem. It is so named because it lasts for the duration of one quarter of a whole note. Also called a crochet.
A series of quarter notes on the staff looks something like this:
/\
---| /-----------------------------------------------------
|/ |
---/------------------*------------|-----------------------
/| * | * |
-/-|/\---------|-----|-----|-------|-----------------------
| | | | | | *
|--|--|--------|-----|-----|-------------------------------
\ | | | |
--\|_/-----------------------------------------------------
|
\|