My personal definition of Endgame is when the pieces for either side total 8 points or less.  Thus, each player may have a rook at most as far as heavy pieces are concerned, and one minor piece in addition to the rook.  One of the most frequently seen endings which fits this definition is rook and bishop with pawns vs rook and knight with pawns.

I was glad to see Glenn Flear's book come out in 2007 titled Practical Endgame Play because it covered endgames which fit my defintion, and a few more types as well.  

When does the endgame tend to begin in terms of move number?  I have noticed chess games typically reach an ending with 8 points or less worth of material in pieces on a move number in the 30s.

My investigation of endings has benefitted me greatly, however...

Although at one time I was heavily dedicated to studying endings, I have decided to study endings and tactics in equal proportion.  My reasoning is that either a well played ending or a well played tactic can basically end the game just the same.