By and large, timber
is correct: each state has its own bar exam. In order to practice law in the US, a lawyer must take a bar exam somewhere. Some states permit lawyers who have practiced a certain number of years (I think it's five) in another state to waive the bar exam requirement, but many states don't allow this.
Bar exams differ from state to state both in content and pass rates. When I took the Illinois bar exam, the pass rate was around 90% (it has gotten tougher, so I'm told). California's bar (reputedly the toughest bar exam in the country) has a pass rate in the 50% range. In 2002, the five toughest and five easiest state bar exams were:
Rhode Island 57%
Kansas 82% (t)
Minnesota 82% (t)
North Dakota 85%
New Mexico 86%
South Dakota 93%
There is no separate federal bar exam. Anyone admitted to practice in a state is permitted to practice in the federal courts in that state. Likewise, I've never heard of any special exam to practice in certain courts, with one exception: patent law.
As for what it's like actually practicing law, that question is too broad to be answered. There are thousands of lawyers in the US, all with different practice areas, in different jurisdictions, with different firms. Perhaps if you narrow your question, pragmatic
, to focus on some area in which you are particularly interested.