Parole is a system of release from prison, for prisoners who have served part of their sentences and whom the parole board deem ready for early release. (Since 1984, federal prisoners whose crimes pre-date November 1, 1987, cannot attain parole.) Probation, on the other hand, can be granted directly after conviction, and given to defendants who have not served any time at all. Both parole and probation can be revoked if the recipient commits a crime or otherwise violates the conditions of their release. Here’s some more information on parole and probation.
People on parole, having been released from a prison, have committed felonies (generally, only felons are sent to prison; those with misdemeanor sentences are sent to local jails). Most of the time, parole is not a right; instead, whether to grant it is up to a group of officials (not necessarily judges), who have complete discretion as to whether a prisoner is fit for parole. Parole always comes with “conditions,” or behaviors that the parolee must follow in order to remain out of prison, such as regularly meeting with a parole officer, working, and not committing further crimes.
Probationers are typically subject to similar conditions, which may also include staying away from specified individuals (such as gang members). Both parolees and probationers may lose their freedoms if they violate these conditions. Each is entitled to a hearing on the matter, and if the board or judge determine that the violation occurred, the person can be sent back to prison or sent to jail; or allowed to remain at liberty with additional conditions. In most states and in the federal system, counsel will be appointed for those who cannot afford to hire an attorney.