Sometimes the definitions for evaluation and calculation are used interchangeably, but they are quite different things, and understanding what they mean is the first step to improving your thinking process during a game.
Evaluation: You evaluate a position to determine the strengths and weaknesses of both sides. Based on your evaluation you develop a short term plan to either attack your opponents weaknesses or fix your own. Dan Heisman defines evaluation as determining which side stands better.
Analysis: The generation of candidate moves and the response to each. For example, “If I move here, and he moves there what would be my response?”
It is important to put these to together and perform an evaluation of the position at the end of the line of analysis you have performed to see if one candidate move is better than another. I have yet to master this integration of the two, but doing this is the hallmark of strong players.