You're thinking of LHD's/LHA's/etc. They are Navy ships, but the Marines launch their 46's/53's/AH & UH 1's/Harriers off them. They transport troops and equipment for amphibious operations. They also carry LCAC's (hovercraft), amphibious vehicles, etc. within their hull. They are really impressive ships and vital for the ARG (amphibious ready group).
Couple side notes: It takes 3-5 amphib. ships to transport a MEU and 12-15 to transport a MEF size MAGTF. (Courtesy of Expeditionary Ops pub from TBS! And I thought they wouldn't ever come in handy!)
Don't let these other guys discourage you. Good question --- compared to some!