Center return: Push the handle one direction to move cylinder out; other direction to move cylinder in. Spring always returns to center, locking the cylinder in fixed position when you let go.
Float detent: Same as center return except if you push it past a detent in one direction it goes into float mode where the cylinder moves freely in both directions. Once in float mode, it will hold itself there until you pull it back past the detent. Same as the loader boom float.
Continuous flow detent: same as float detent except instead of going into float, the detent position provides continuous flow for running a hydraulic motor, etcln
Edit: And TheOldHokie knows a lot more about this stuff than I do.