Louisa is correct, shatzi, your question is a bit broad. But let me try to give you some impressions from two river cruises and four land tours.
The river cruises obviously stay close to the river, and touring away from the river is usually an optional tour when available at all. Land tours are not so restricted and give you, perhaps, a better view of "life in the countryside". There are also some "must see" tourist sites that are not convenient to the river.
Another difference is that you have to change hotels, more or less frequently, with the land tour while on the river cruise your hotel goes with you. I will compare two of the land tours I have taken with GCT to show how the hotel changes differ. On the tour of Italy, which was called Amalfi Coast and Tuscany when we went but is now reversed, we included the Rome pre-trip extension. We spent 5 to 7 nights in each of three hotels, plus one night back in Rome just before the flight home. So we had only three "check-outs" with which to contend. The contrast was the America's Majestic National Parks tour, with the Badlands pre-trip extension. After about five nights at the same hotel in Rapid City, SD, we went "on the road". Except for three "two-night stays", we changed hotels every day.
There used to be a third option, the extended stay on the Costa del Sol in Spain. There we stayed three weeks in the same hotel room. But that tour has been dropped by GCT for 2012.
Since all of the land tours are different, both in itinerary and in amenities, it's really hard to generalize. But these thoughts are my "basic differences" that come to mind.