A grace note is a note usually displayed as a smaller sized note which does not have duration of its own. The duration of gracings is borrowed from the next normal-sized note (to which it usually connected with a slur). Grace notes are inserted with the grace note menu
Grace notes are by default always shown with stick pointing up. Of course you can specify stem-direction manually. Hitherto you'll use menu-others.
It is possible to cluster two or more grace-notes to form double or triple gracing.
Although grace notes are of the same size as cue notes, they serve different goals, and are interpreted very differently.
Grace notes are also referred to as accacciatura (short grace, dash through the note) and appoggiatura (long grace) in Italian.
How grace notes are played depends on settings in the playback dialog.