What's the difference between Snap and Osnap on AUTOCAD?

i am totally agree with the answer of Mr. Vaqar Ali Sayyed.

SNAP : Snap mode restricts the movement of the crosshairs to intervals that you define. When Snap mode is on, the cursor seems to adhere, or "snap," to an invisible rectangular grid. Snap is useful for specifying precise points with the arrow keys or the pointing device.

OSNAP : This not to be confused with OSNAP, or "Object Snap" which adheres to specified entity points (endpoint, midpoint, Center, intersection, Quadrant, etc)."

snap command for single line / O snap for whole objects 

I apologize for the answer I leave the answer  to the specialists experts in this the field that's not my area.