, an offensive move. It seems to have come into usage because the defensive player is, when the move is performed correctly, used as a post
There are many kinds of postup moves, and two different areas of the court referred to as the "post". The high post is 15-20 feet from the basket, usually on the wing or on the baseline. It is used mostly by players with jump-shooting ability, usually guards, though some big men (Patrick Ewing is one example) operate out of the high post. The low post is just outside of the painted area under the basket, between 5 and 10 feet away. This is where the centers and power forwards usually set up. Shaquille O'Neal is the prime example, and the dominant low-post player of his time. The area inbetween the two is sometimes called the mid-post.
In the low post, the offensive player usually has his back to the defender, and in the high post he will usually face up. In the high post, there are three basic options: drive to the basket, fake a drive and then shoot, or step back and shoot. In the low post, the ideal outcome is an uncontested dunk, but if defended well, any professional-level player can resort to a hook shot or a fadeaway jumper. Hakeem Olajuwon preferred to fade away, and became so automatic with that shot that he scored 26,000 NBA points, and counting.
Just as there is post offense, there is also post defense. Players such as Charles Oakley and Horace Grant have built entire careers out of being able to consistently frustrate low-post scorers. A team with excellent defensive players can use a tactic called "fronting the post", where the defender actually steps in front of his man. This makes it much more difficult to throw the entry pass, and if the pass is delivered, another player comes over to double-team. The drawback to this is that if the double-team is late to arrive, the offensive player will have no one in front of him when he rolls to the basket, and most likely will throw down an uncontested dunk. Probably the best man-to-man post strategy is to try to foul the offensive player without calling the referee's attention to it. Plant your feet firmly and become an unmovable, elbow-throwing object.