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Federal law is a step above Congress. It's actually more important.
It's a law in all of the United States, as well as Canada not to murder people. Congress, or Provincial courts can't change that.

Roe v. Wade was a precedent made by the Supreme Court. Because they decided one way all other lawyers of a lower level had to use that as the example. They can't argue against the Supreme Court.
The Supreme Court also had to make a decision one way or another so they looked at all the information and the majority of them decided that this was the best course of action. Basically, most of the 7 or 9 people involved decided that it was alright for a child to die so now no lawyer can go against that and children are dying everywhere.

Yes, it seems taht the supreme court has a little more power than the others. Once they rule in favor of something, that law becomes set in stone until another ruling. And I don't beleive congress or the president can overturn those rulings, can they? Only another ruling can.