What does the 7000 mean in D7000? Or in any Nikon SLR camera?

I've been trying to google it, but I'm just having no luck figuring out what those numbers mean.

Ex. D90, D7000, D3S

There's some kind of logic, but in the end it really does not matter.

At present the D90 was replaced by the D7000.

The D3s is Nikon's professional full frame dSLR (as is the D3X and D3)

The D3100 and D5100 are entry level (under $1, 000) cameras that include a lens

You just have to read about those camera models and in the copy, there will be specifications that will help you 1) decide which body you need for your type or style photography and 2) the one that fits into your budget.

Most photographers find themselves making compromises, balancing price with image quality

In a perfect world, we could own a Nikon D3X (studio camera), D3s (editorial and sports camera), D300s (a cropped sensor camera for those of us who need the extra reach they give on standard long lenses)

After you have spent a decade using a Nikon system, you will find all this a minor issue.

Why?
It is simply a model name and within the Nikon range of cameras it lets you know the general position within their extensive line of cameras. In Nikon, the more zeros, the further down the product line. It is also likely (Not a nikon guy and didn't bother to check) that it has different names in different markets (Japan, USA, Europe).

The name is a pure product of marketing research and, other than the possibility of the "D", for Digital, it has no more or less connection to the product than any other product name.

Of all the features to be concerned about, the name is not even on my radar.

None. Nikon is using witchcraft to name their dSLRs. There's no pattern in their numbering system unlike Canon and Sony.