A key cel is drawn by a key animator.  They're 
typically the first cel (A1, B1, C1, etc..), and 
the last cel (A3 END, B17e, etc...) in a 
sequence.  Midsequence key cels usually have 
their numbers circled/triangled or have the 
numbers circled on the sketch.  If there's only 
one cel in a sequence, you can count on it being 
key (A1 END, C1e, etc...).

An end cel is the last cel in the sequence (C37 
END, A9 END, B1 e, etc...).  Not to be confused 
with an ending cel a.k.a. ED cel (any cel shown 
during the ending credits).

A book cel involves a (partial) BG layer 
covering the front of a cel layer instead of 
behind.  These are often background cutouts to 
place scenery or what not in front of characters 
(like tree or mountain cutouts).