The Department of Transportation's Federal Highway Administration has a page on its website devoted to U.S. traffic sign meanings. Wikipedia has an extensive article that covers national signs and some local signs, too.
The Federal Highway Administration breaks down the traffic signs into major categories including warning, regulatory, temporary, guidance and pedestrian and bicycle signs. Remaining categories are motorist services and recreation plus railroad and light rail signs. The Web page also include mandatory movement and a couple of lesser categories of signs.
Wikipedia offers dozens of traffic signs and meanings, focusing primarily on regulatory and warning signs.
Practice driving tests are available for free at the websites USA Traffic Signs and Road Traffic Signs, both of which offer tests of varying difficulty.