Grand vs. Petit
Here is a very brief overview of certain aspects of the jury system in the USA, pertinent to the Michael Jackson case.
The Sixth and Seventh Amendments to the Constitution of the United States establishes the right to a jury trial, in all federal criminal and certain civil cases.
There are three types of juries in the USA. The standard, and most common, is the "petit jury". Petit juries can cover both criminal and civil cases, and contain between 5-12 jurors.
One of the other two types of jury is the "grand jury". These can contain up to 23 people, and can act as either the charging grand jury (which decides if there is enough evidence for a trial to take place) and an investigatory grand jury (which determines whether evidence should be gathered to render a prosecution possible).
Michael Jackson faced a grand jury in April, which decided that there was enough evidence to go to trial.
He will then face a petit jury, which will decide his guilt or innocence.