I use a Garmin eTrex Legend. It's the one with black and white screen and I would prefer the color. Color shows up better in most conditions.
I'm not sure what you mean by the difference between it and a map. You really do need to know how to use a map, as well as know how to use a compass. This model has a map screen... if that helps you - also a compass screen.
I use mine mostly on Searches. When we have our subject's coordinates (and they remain stationary) it is ideal. It'll take us right to the spot if satellite coverage is decent. Sometimes out in the woods, the density of the trees will interfere a little.
When the subject's coordinates are unknown, we use it to track areas where we've already searched. That information is uploadable to our Sheriff's department mapping system on the computer.
When my hubby and I are out in the woods, scoping out hunting blinds, I will set waypoints in the desirable places, then come home to upload them to my own mapping software. The topo map gives him a birdseye view of the roads, rivers, lakes and the topography. He usually knows all that anyway... but it's interesting to me.
It's usefulness really depends on what you want to do with it. If you have specific questions, you can pm me.
SARgal