The federal criminal system is in many ways like the state system, but it has some very significant differences. One difference is that the typical United States Attorney's office, responsible for prosecuting federal criminal charges, has significantly more time and resources to direct to any given prosecution than would a state prosecutor.
Federal prosecutors also typically have better academic credentials than state prosecutors, and many have a great deal of latitude in selecting the cases they wish to prosecute through the federal courts. Save for crimes which occur on federal land, those cases which fall exclusively to the jurisdiction of a federal prosecutor tend to be of an interstate nature, and are more likely than a state prosecution to be legally and factually complex.
As a result, federal criminal defense tends to involve cases which are more difficult to defend, and the cost of defense is often very high. In each federal jurisdiction there is also a Federal Defender's office, which can provide legal representation to indigent defendants.