In music notation, a grace note (occasionally called an acciaccatura) is a type of ornamentation drawn as small eighth note with a slash through its stem and flag positioned just before a note, either a half step or a whole step higher on the staff. It is not factored in when calculating the time signature for the current measure.
A grace note is played on the beat as a very, very short note slurred into the note it precedes, typically the length of a thirty-second note which is "stolen" from the main note. On the staff, it looks something like this (complete with time signature):
/\
---| /------------------------|------------------|-------
|/ | | |
---/-----------*-------|X-----|-----|------------|-------
/| 2 | . | | | |
-/-|/\--------|----------*----|-----|-------|----|-------
| | | 4 | | | * |X | |
|--|--|-------|---------|-----|---------.---|----|-------
\ | | | | * |
--\|_/------------------|-----|------------------|-------
|
\|