Probation is an action taken by a church leader or council that limits some privileges of church membership. Not all probation is negative; quite often, it is merely counsel to attend the temple more, read select scriptures, or pray more frequently and fervently. The only difference between informal and formal probation as far as I understand it is that formal probation occurs by a disciplinary council.
Disfellowshipment is a suspension of participation in church activities, callings, ordinances, etc. -- the public or "outward" things. Usually these individuals should still attend sacrament meetings. And, like probation, conditions of disfellowshipment may be positive.
Excommunication means the individual is no longer a member of the Church; this is actually quite rare.
In all of this, Church leaders remember that "the Lord sees weaknesses differently than He does rebellion. Whereas the Lord warns that unrepented rebellion will bring punishment, when the Lord speaks of weaknesses, it is always with mercy." -- Richard G. Scott