Definition: Par is the number of strokes an expert golfer is expected to need to complete an individual hole, or all the holes on a golf course.
The value assigned to represent par for an individual hole is always comprised of two putts and the number of strokes it should take to reach the green. Holes typically are listed as par-3, par-4 or par-5, although par-6 is also occasionally encountered. A par-4 hole is going to be longer than a par-3 hole, and a par-5 longer than a par-4 (with rare exceptions).
On a par-3, an expert golfer is expected to need only one stroke to reach the green, followed by two putts. On a par-4, he should need two strokes to reach the green, followed by two putts; and so on.
For 18 holes of golf, the par is the total number of strokes an expert golfer is expected to require to complete the course. Most full-size golf courses range from pars of 69 to 74, with par-70, par-71 and par-72 courses most common.
“Par” is also used to describe a golfer’s peformance on an individual hole or for a complete round of golf.
If you complete a par-4 hole having used four strokes, then you are said to have “parred the hole.” This is also referred to as being “even-par” or “level par.” If you take five strokes to play a par-4 hole, then you are 1-over par for that hole; if you take three strokes on a par-4, you are 1-under par on that hole. Same applies to 18-hole scores: If the golf course’s par is 72, and you shoot 85, you are 13-over par; if you shoot 68, you are 4-under par.
source : http://golf.about.com/cs/golfterms/g/bldef_par.htm