hammos1 is correct, but to flesh it out a bit:
When sentences are handed out, one practice is to sentence the person to some period of time, then to "suspend" a certain portion of the sentence. During the suspended portion of the sentence, that person may return home and go on with his or her life, but under as term of probation (proving) and must follow a much stricter set of rules than the average citizen, with a return to jail for the original sentence duration a common penalty for breaking those rules.
For example, a person who is convicted of battery resulting from a drunken brawl may be sentenced to two years in prison with 23 months suspended, meaning that the person will only spend 30 days in jail. However, for some period of time following their release (and it is not limited to the 23 months they "got off"--the probationary period may extend beyond the period of the jail sentence), they may be required to enter an alcoholic treatment program, refrain from drinking any alcoholic beverages, avoid any misdemeanor charges, avoid associating with their drunken buddies, or a number of other rules. So, if he or she gets picked up for yelling at his or her spouse or if they are charged with reckless driving, even if no alcohol was involved, or if they go to a Superbowl party with their old drinking buddies, even if they drink only water, they can find themselves back in jail serving out their 23 months, on the first charge. The probationary period is also used for people who have been paroled from prison (where the "no association with your crummy criminal friends" rule is more often imposed).
Probation for youth offenses can be extended indefinitely from the first offense until the child reaches legal age, regardless of the sentencing for whatever law was broken. I am not sure how long probation may be imposed on an adult, either under the suspended sentence situation or under the parole situation. I am pretty sure that it can extend beyond the period of their original jail sentence, but I do not know whether it can be extended indefinitely.
Generally, each offender is assigned to a case worker (probation officer) with whom they must meet on a regular basis to provide evidence that they are satisfying the conditions of the probation.