OS X Lion = 10.7
OS X Mt Lion = 10.8
OS X Mavericks = 10.9
Usually a Mac that can run Snow Leopard can run Lion and usually a Mac that can run Mt Lion can run Mavericks. Lion and/or Mt Lion will cost you $20 each, whereas, Mavericks is free. If your Mac can run the OS, you can jump from Snow Leopard to any of the three and not have to buy and install the intervening version of OS X.
System requirements for OS X Snow Leopard & OS X Lion
System requirements for OS X Mountain Lion
System requirements for OS X Mavericks