The first 5-10 moves of a chess game is called the opening. The part of the game in which most of the pieces are already off the board is called the endgame. The part of the game in between is called the middlegame.
middlegame

wow luke ... ur really into the studying now ... thats good :)
i need 2 startdoing some aswell ...!!!

yeah i know but see my average opponent their rating ... :S its low... and when i play high people i keep losing ... so i gotta work on that 1

The middlegame is a huge subject and not easily categorized. It's not easy to study. Here's some of my favorites.
The Art of the Middlegame - Keres and Kotov
Art of Attack in Chess - Vukovic
Pawn Structure Chess - Soltis
Positional Ideas in Chess - John Love (good luck finding it)
Successful understanding of the major principals of these three will get you to at least a class B level. It will hurt though. I remember making lots of goofy moves as I learned this stuff. Keep up on your tactics and endgame study at the same time.
What is the middlegame?
Is that like the tactics?