There are three, actually.
The 70-300 f/4.5-5.6 G is the cheap, made-in-China one that sells for about $125 in the US. It's a kit lens that's okay as long as you keep it pretty well stopped down at the longer focal lengths and otherwise don't expect too much of it. There are quite a few of these on
Ebay, probably for sale by owners who got them as kit lenses and are ready to step up to something a bit better.
Then, there's the 70-300 f/4-5.6 ED lens that sells for around $325.00, I think. I don't have any experience with this lens, but it's safe to assume that it performs considerably better than the aforementioned cheapo. ED (Extra-low Dispersion) glass is a special formulation that reduces chromatic aberrations caused by the fact that different wavelengths of light (colors) converge at different points after being refracted through a lens.
Finally, there's the new (introduced last summer) 70-300 f/4.5-5.6 VR lens. This one costs around $525 or so and features ED glass, IF (internal focusing, meaning that the lens does not change length as it's focused), and VR vibration reduction technology.