I will not go into the details since Cameron Fuller and Kerry Meyler have already done so and very well I must add. However, there is one (small) topic which needs clarification since people tend to think like this:
Agentless Monitoring = Licenseless Monitoring
But that is not the case since every monitored system needs a license. Whether an Agent is installed or not. Want to know more about licensing? Check out this previous posting of mine.