In dynamics music notation, a crescendo is drawn as an elongated less-than (<) symbol positioned beneath the staff or in the middle of a grand staff, although sometimes it is abbreviated as "cresc." instead. It indicates that the loudness of the music should increase gradually and smoothly from the point where the crescendo begins to where it ends. The word means "increasing" in Italian.
Usually, a specific dynamics marking will be indicated at the end of the crescendo to tell the musician how loud the music should be when it ends.
When drawn on the staff, crescendo looks something like this (complete with time signature, a quarter note scale, and piano and forte markings):
/\
---| /------------------------------|--------------------||
|/ | | ||
---/--------------------------------|---|----|-----------||
/| 4 | | | | * ||
-/-|/\----------------------|----|--|---|----|---*---|---||
| | | 4 | | | | | * | | ||
|--|--|----------|-----|----|----|--|--*--------|----|---||
\ | | | | | * | | | ||
--\|_/-----------|-----|---*--------|-----------|--------||
| | *
\| -*-- ___________
____________------------
p < ____________ f
------------___________