They're not that different. Parole, which is what guys from state prison get, is a situation where guys get out early, but aren't necessarily free as they have stipulations they must adhere to. Violations of their parole can result in their parole being revoked and the parolee can be returned to prison to finish out their original sentence. Common stipulations are no drugs/alcohol, no contact with victims, no new crimes, weekly visits with their Parole/Probation Officer.
Probation is pretty much the same thing but comes from the county. It, too, has the same stipulations attached and can also be revoked. In the situation of a plea bargain, a revocation of probation can result in that person being sent to jail to do the time that is associated with their crime. Remember, a plea bargain is still a plea of guilt.
The real differences between the two besides who (state or county) dolled them out is the crimes associated with them and the duration (time) of their sentences. Here in Oregon, anybody sentenced to a correctional facility for up to a year will find themselves in a county jail (eligible for probation). A year and a day or more and they will become the ward of the Department of Corrections and find themselves in a state prison (eligible for parole).