Unlike the prosecution, the defense in a criminal trial need not put on any case at all. This is because the prosecution has the burden of proof. To convict someone, the prosecution must prove the defendant is guilty beyond a reasonable doubt. Here, the prosecution must prove, beyond a reasonable doubt, that Dr. Murray committed involuntary manslaughter against Michael Jackson. (See prior posts here and here for more about the involuntary manslaughter charges.)
Once the prosecution has presented all the evidence it believes proves the defendant guilty, it rests its case. The defense then has the option of putting on its own case. However, the defendant need not do anything. He may simply attempt to poke holes in the prosecution's case, and argue to the jury that the prosecution did not meet its burden of proof.
If the jury has a reasonble doubt, whether based on weaknesses in the prosecution's case or the strength of the defense evidence or both, it must acquit. Although they need not do so, Dr. Conrad Murray's defense attorneys have indicated they intend to call witnesses in his defense. If Dr. Murray chose to testify, that would also happen during the defense case.