Closing arguments are often the most exciting part of a trial because this is the only time that the lawyers are permitted to "argue." They must confine their arguments to the evidence that was admitted at trial, or reasonable inferences that can be made from that evidence. The lawyers must also tailor their arguments to the law that the jury is instructed to apply.
Procedurally, the prosecution goes first. This is because the prosecution has the burden of proof. The prosecution must prove the defendant guilty beyond a reasonable doubt. During closing argument, the prosecutor will explain how the evidence proves the defendant guilty.
The defense then argues why the evidence does not prove the defendant's guilt beyond a reasonable doubt. The defense attorney may even argue that the evidence demonstrates innocence, although such argument is rare. Usually, the defense strategy is to poke holes in the prosecution's case, bit by bit, to show there is a reasonable doubt about guilt.
Finally, the prosecution is permitted a rebuttal argument to respond to the defense. Again, because the prosecution has the burden of proof, they are permitted the last word.
In the end, however, the jury's decision must be based on the evidence and the law. The attorneys' statements and arguments are not evidence. They are merely provided as an attempt to guide the jury in its decision.