The federal tax evasion statute, 26 USC § 7201 is one example. It states: “Any person who willfully attempts in any manner to evade or defeat any tax imposed by this title or the payment thereof shall, in addition to other penalties provided by law, be guilty of a felony and, upon conviction thereof, shall be fined not more than $100,000 ($500,000 in the case of a corporation), or imprisoned not more than 5 years, or both, together with the costs of prosecution.” Of course, a lot of tax evaders do not get the full 5 years on just a single count of tax evasion, but the statute does allow for it.
A federal crime is one that violates federal criminal law. Most federal crimes are those that fall into one of three general categories: (1) acts that violate national security or that are committed against the federal government itself; (2) acts that are committed on federal lands, ocean areas under the control of the United States or on ships that are U.S. flagged; and (3) acts that have some element of interstate activity. There are some federal crimes that do not fall into one of those categories but I think that will give you a sense of the sort of things that federal crimes target.
I might be able to give you some more examples of laws that might fit what you are looking for if you explain why you are asking the question. Are you writing a book where this is important? Is there a situation you are involved in that raises this issue? Something else? Knowing that would help narrow things down for a search.

