Activity diagrams are used for to show a subprocess of a system by figures. Big black dot means the start and the Black dot and a circle outside it means end. Also the arrows represent yes and no answer to the process.That means if yes process done if no can not be done. The diamond are the process steps.
We still continue on the example we examined at last two articles. Here are the some examples from the dynamic web site system:

Activity diagram for use case of Adminstrator to send and receive mails.



Activity diagram for use case of all users to post messages.



Activity diagram for use case of Student to add comments.