What are the difference between free float, total float and what is different between Float and Slack?

Free Float (FF) = the maximum amount of time an activity can be delayed without delaying the early start of any of its succeeding activities.

Early start  – Early Finish = Free Float

Total Float (TF) = the maximum amount of time an activity can be delayed from its early start without delaying the entire project. Late Finish – Early Finish = Total Float

Slack and float are the same thing. Slack or Float provide flexibility in the project schedule. 

Free float is the time an activity can be delayed without delaying the successor activity.Whereas total float is the time an activity can be delayed without delaying the project.

Slack allows an activity to start after its scheduled time whereas float allows allows the activity to take longer than planned.