Ornaments

Ornaments are added to sheet music to add a flourish to the notes played.

These two kinds of grace notes add a note before the main one:

 acciaccatura / crushed note - play the extra note briefly, with emphasis on the main note
 appoggiatura - play the extra note, then the main note (equal value)

These add a note afterwards:

 mordent - play the note, one above, then the note again
 lower mordent (inverted) - play the note, one below, then the note again
 trill - quickly alternate between the note and one above 

And these turns add even more notes:

 turn - play note above, note, note below, note
 turn after - play note, then note above, note, note below, note
 inverted turn - play note below, note, note above, note

Read more: Wikipedia