Bowling the over.

The bowling of the six balls in an over involves a tactical decision by the bowler on what each of the six balls is designed to achieve. Each ball is bowled in the context of the balls around it, rather than in isolation. The sequence is designed to set up the batsman and then try to get him out.
Fast Bowling
A typical sequence of attack by a fast bowler is to pitch the ball on a good length just outside the batsman's off stump. If he can achieve some outswing, all the better. The goal here is to create some doubt in the batsman's mind as to whether the ball might hit the wicket or not. If the batsman has any doubts, he must play at the ball, and the more he plays the more likely he is to make a mistake and be caught by a fielder or play on to the wicket. A secondary goal is if the batsman is skilled enough to judge the line and let balls outside off stump go. In this case, persisting with this line will eventually test the patience of the batsman, who may be tempted to play an attacking shot and risk getting out.

Once the batsman has settled into a rhythm of playing or leaving balls outside his off stump - either towards the end of an over, or after more than one over - the bowler might try a sudden variation of line and length. A good example would be a yorker aimed at the wicket. A yorker is difficult enough to play at the best of times, but if the batsman has become weary or complacent and is surprised by it, it becomes even more dangerous. Other possibilities include changing to inswing, using cut, bowling a slower ball to induce a mistimed shot, or pitching short and aiming at the batsman to force a back foot defensive that may result in a misplaced hit into the air.

If the batsman is playing shots at balls pitched outside off stump, the bowler may try pushing the ball wider, tempting the batsman to play farther from his body. This increases the chances of getting an edge from either side of the bat, resulting in the possibilities of being caught by the wicket-keeper or slips, or playing on.

Generally, the primary goal of a fast bowler is not to aim at the batsman's wicket in an attempt to hit it, because this provokes an obvious response from the batsman - he must defend the wicket. It is more productive for the bowler to put doubt into the batsman's mind over whether or not he needs to play at a ball, and entice him to try to score runs, with the goal of inducing a mistimed shot and producing a catch.

A few other tactics used by fast bowlers:

Bowlers may attack the leg stump, directing most balls towards the leg stump or even into the batsman's legs. The batsman must play at these balls, to avoid being bowled or LBW. This means the bat is protecting the leg stump, leaving the off side of the wicket unprotected. A variation ball with outswing, seam, or a leg cutter can be devastating, either hitting middle or off stump, or catching the outside edge of the bat for a catch behind the wicket.
If a batsman is being aggressive and playing a lot of front foot drives, the bowler may respond by pitching the ball shorter, so it bounces higher as it reaches the batsman. This makes front foot shots less attractive and can push the batsman on to his back foot. A predominantly front foot batsman will find it harder to score effectively from the back foot, and will need to spend more time on defensive batting. This can upset his rhythm and force errors of judgement and timing.
Short pitched bowling can be used to upset the rhythm of a batsman who is scoring well, or who is trying to settle into his innings. A good batsman will usually evade a short ball easily, but it disrupts the flow of facing balls pitching closer and prevents the scoring of easy runs. Poorer batsmen can be intimidated by short bowling, especially if aimed at the body, unsettling their mental state. A short ball aimed at the body at waist to shoulder height is tricky to deal with, and can provoke a poor defensive shot in an unprepared batsman, resulting in the ball flying off the bat or the batsman's gloves for a catch. A bowler will not bowl many short pitched balls because once a batsman is prepared for them they become easier to evade or otherwise play safely.
On the other hand, if a bowler successfully pushes a batsman on to the back foot with short pitched bowling, and keeps him playing back foot shots for a while, he can follow-up with a yorker. If the batsman has relaxed into expecting a short pitched ball, this can be devastating, because the speed needed to react and bring the bat down fast enough to defend against a yorker will be lost in his slowed reaction time.
And then bowlers sometimes defy these conventions and bowl an unexpected delivery on the first ball of a new over. If the previous over was all good length, outside off stump, the batsman may expect the next over to start the same way. Switching to a yorker or short pitched ball to start the over can be a surprise that the batsman may struggle to deal with.

Note that these tactics can be interrupted if the batsman scores runs and the non-striker comes to the striker's end. This is why batsmen like to swap ends frequently if they can. Part of the bowler's goal in preventing the batsmen from scoring runs is simply to keep one batsman on strike so he can work a tactical sequence of balls against him (as well as preventing the batsman's side from accumulating runs, of course).