It is divided into two separate house (the Senate and the House of Representatives).
It is not *less* powerful than the Executive branch (the President); it is simply *different* powers. It is not responsible for conducting trials (unless you are referring to impeachment of the President); that role belongs to the Judicial branch (the Supreme Court). As for enforcing the law, that seems to be a bit of a murky question; Congress is responsible for certain areas, but not over the country in general. (Basically, each of the three branches has powers that are able to counter actions by the other two branches -- the "checks and balances" function.)
