That is a very good question. I'll try to make it clear.
The Present Continuous Tense is the one that most truly expresses an action in the present time, an action that is taking place NOW.
"Mary is playing the piano" or "I am writing on the blackboard" means that these actions are going on at this very moment.
The Simple Present Tense is generally used for an action that is done regularly, for an action that happens, perhaps, every day. So you often find words like USUALLY, GENERALLY, EVERY, DAY, SOMETIMES, ALWAYS, etc., used with it.
E.g. "I am dancing" - PRESENT CONTINUOUS; "I dance every day" - SYMPLY PRESENT
So remember:
THE S.PR.T. IS USED FOR A REPEATED OR HABITUAL ACTION.
THE PR.C.T. IS USED FOR AN ACTION THAT STILL CONTINUES.
P.S.: If you have will get any questions - ask them. I'll try to answer all them)))