Length, over pitched, good length, back of a length, half-volley

The length of a delivery is judged by how far down the pitch the ball bounces on its way towards the striker and its effectiveness is also governed by the line on which the ball is bowled.
A good-length ball is one that has the striker in two minds - uncertain whether to play forward or back to it. A good length to one striker might be very different from the good length to another, depending on the height of the individual and thus how great his reach is.
Sometimes the bowler will fail to achieve that good length and over pitch the ball. An over pitched delivery, being that fraction nearer to the striker, is often easier to hit for runs.
A ball described as short of a length will pitch further away from the striker. The object is to force him on the back foot by making the ball rise towards his chest. In fending it off the striker might perhaps give a catch to one of the close fielders. If the bowler errs in his line, however, he can also present the striker with an easy boundary shot off such a ball.
A recent introduction into cricket terminology is the phrase back of a length, which describes a ball pitching slightly shorter than that bowler's good-length delivery and thus rising a little higher than it otherwise would.
A bouncer (sometimes called a bumper) is similar to the short one, but pitched even further away from the striker, with the object of making it rise towards the striker's head. A reasonably competent batsman should not be too intimidated by this and a bouncer that is slightly off line can often be hooked to the boundary.
Bowlers try not to bowl half-volleys - easy-to-drive deliveries that pitch near to the bat, or long-hops, which are short-pitched balls that lack venom and are easy to hit away.