The president can veto or reject, bills, passed by the congress. This is a check on the power of congress, but congress can override a veto. if both the houses pass the bill again by 2/3 it now becomes a law!
the president has the power to choose who the judges, ambassadors, and other important officals. The senate can vote ot reject any of these choices.
If the president has been convicted of any crime he/she can be brought to trial by process called impeachment. The senate acts as a judge and jury. A two third vote is necessary. In such cases the house representitaves brings the charges against the president.