How state Diagram are different from the Activity Diagram?

Answer 1

They are not as they both have the same features it just provides a better understanding when using one diagram in conjunction with the other.

State machine diagram shows how state of some object or system could change, so transitions on the diagram are from one state to another state.

Activity diagram shows set of actions and how one action could lead to another.

In simple cases these diagrams might look like reverse of each other but in practice you need to select whether the diagram is about discrete number of states of some object or is it about organization of actions and activities possibly related to many different objects.

Answer 2

State diagram shows the object undergoing a process. It gives a clear picture of the changes in the object's state in this process.

e.g: ATM withdraw

Card object state: Checking, Approving, Rejecting

Activity diagram is a fancy flow chart which shows the flow of activity of a process.

e.g: ATM withdraw

Withdraw activity: Insert Card, Enter PIN, Check balance, with draw money, get card

State chart shows the dynamic behavior of an object.

Activity diagram shows the workflow behavior of an operation as set of actions