Turnips are really the roots that are best eaten small (i.e. 2" diameter) and don't really store well after harvested. You'll want to eat them through the growing season, don't leave them to grow huge as they get woody quite easily. Pick them young and add them to salads, stirfries, etc. Some folks refer to them as "summer turnips".
Rutabagas are the typical winter storage root that you'll see in "stew packs" with cabbage and carrots at the co-op or farm market in the fall. They're usually harvested around softball-size, and hang on in the root cellar or back of the fridge for a loooong time. For some reason, however, the name rutabaga has fallen by the wayside and most folks use the name turnip for both turnips and rutabagas - just to be confusing!