Answer:
The executive power is led by the President, and is in charge of executing the law, and implementing government policy. The executive is also in charge of the armed forces.
The legislative power consists of the Senate and the House of Representatives, and is in charge of creating the laws. It also checks the power of the President, and has the faculty of impeachment (removing the President from office).
Finally, the Judicial branch consists of the courts, the most important being the Supreme Court. It is in charge of interpreting the law and delivering justice.
Each branch checks on each other power. For example, the Supreme Court has the power of judicial review: it checks that the laws passed by Congress are constitutional, and in case they are not, these laws become null.
Another example is the Presidents' veto power: laws only become law when the president signs them, and the President has the power to refuse to sign (to veto) any law (only once) that comes from Congress.