In music notation, a bar-line is a thin vertical line drawn through a staff to indicate the separation between two measures (or bars). It is drawn from the top horizontal line of the staff to the bottom one. When a grand staff is used, the bar-line is continued between the top and bottom staffs.
It looks something like this (complete with staff, treble clef, time signature, and quarter notes):
/\
---| /-------------------|--|---------------*---|
|/ | | | * | |
---/-------------|---|---|--|-------*--|---|----|
/| 4 | | | | | * | | | |
-/-|/\-------|---|---|--*---|--|---|---|---|----|
| | | 4 | | * | | | | |
|--|--|------|--*-----------|--|---|------------|
\ | | * | | |
--\|_/----------------------|-------------------|
|
\|