What is the difference

In a computer analysis what is the difference between and inaccuracy, a mistake, and a blunder? I know a blunder is the worst but what makes a move fall in each category?

My guess is that any answer you get to this is likely to be overspecific and thus not terribly useful.

Whether a move is considered to be a blunder, a mistake, or an inaccuracy basically depends on exactly how much worse than the best move it was. You can read all about it here:

http://support.chess.com/Knowledgebase/Article/View/153/0/in-the-computer-analysis-whats-the-difference-between-inaccuracy-mistake-and-blunder

That helps but how is the best move determined. I as a beginer sometimes in my mind am playing a good move but it turns out it was horrible.

Hm...sounds like it was horrible to begin with then.

I know where you're coming from; I play them all the time, those awesome grandmaster moves that suddenly stop being awesome grandmaster moves the second a reasonably strong player takes a look at them.

As for determining the best move, the computer evaluates the positions resulting from every possible move (or sequence of moves) according to a number of parameters set by the programmer. (If you want to know more about how this works, this page has a number of good links to get you started: http://verhelst.home.xs4all.nl/chess/programming.html) The problem is that while computers are pretty good at playing chess this way, they suck somewhat horrible at explaining chess. All they can do is give you their evaluation of a given position ("+2.6, white is winning") and the line they consider to be best play for both sides. Looking at the latter can sometimes be useful when they point out a gross tactical mistake, but more often than not you're left with an endless sequence of random moves and a cryptic evaluation.

This is where having human look at a position is greatly superior, even if the computer is technically the stronger player. A human player can not only give you an assessment of the position ("White should be winning"), but also their reasons for thinking so ("White's up a pawn; his rooks dominate the only open file while Black's pieces are awkwardly placed and can't move anywhere without allowing deadly tactics.")

If there's a position or move you don't understand, your best bet might be to ask a stronger player for their opinion or post it on the forums to get some feedback.