As someone accused of a crime, you have a right to know if you're facing a unique charge or one that comes up often. For charges that are filed often, attorneys may have a better idea of how the local courts treat the charges and what you may or may not want to do to help reduce the penalties you could face.
Federal crimes are those that fall under federal jurisdiction. Some crimes, like medical fraud, will be charged at the federal level. It's not always individuals that face charges, either; it can be organizations or business entities, too.
The federal government does not recognize the legalization of marijuana. In Georgia, marijuana is regulated under the state's Controlled Substances Act, even though it is not a scheduled substance.
Perhaps police pulled you over and found an unmarked bottle of pills in your pocket while you were crossing state lines. Perhaps an officer thought you stole something from federal property, but the item was yours and you were carrying it on your person when stepped onto the property. Or, maybe a Sheriff's Deputy caught you in the act of mailing some crystal meth to another state. No matter what the circumstance surrounding your arrest and federal crime allegations were, in the United States, you will remain innocent until proven guilty beyond a reasonable doubt. Until then, you have every right to defend yourself against your charges.