Difference between router and modem-router?

i was using the linksys WAG300n . which had 4 lan ports and 1 dsl port (which looks different from other 4). i just bought a new linksys WRT120n to replace the old one . only after reaching home found that it has 4 lan ports and 1 internet port which looks just like the lan port ! i dont know how to connect the internet line from the adsl splitter to the new router . please help me !