The president may have the voice to change the country, but without congress to back him up, it won't happen. If the president does something bad or something that will affect everybody in the country, like Bush sending the Army over to Iraq, he needs to get approval from congress. So, what I'm saying is that the presedent has the voice, but Congress has the power. Congress can always just disagree with the president. So if Mr.Pres says something stupid, and Congress agrees and makes the preperations to carry it out, they're both to blame. Just a bit of a reminder that we don't like in a dictatorship where the president can do whatever he wants without the supervision of someone with just as much power as him.
Congress passes the laws, Mr. Pres just comes up with them.
Congress decides how to spend our money, if we want to build a giant ring in space to "recreate" the Big Bang, Congress decides if they should give the money to do it or not.
Congress shapes our foreign policy. Mr. Pres may be our Commander in Cheif of our military, but Congress has the power to declare war- not Mr. Pres, though he may suggest it.
Those are the big 3 that affects everyone the most. Just remember that next time you vote.;)