Is the lens compatibilty of the nikon d3000 and the d3100 the same?

I want to buy a new lens and most of the websites say that a particular lens is compatible with a d3000.So if it is compatible with a d3000 will it also be compatible with a d3100?

Yes it will

Yes - neither of these bodies has a focus motor, so if you want autofocus lenses, you need to buy those mark AF-S (Nikon) or HSM (Sigma).

Yup.

Yes. In fact, both cameras are compatible with almost 64 million of the lenses that Nikon has built over the last 50 years. They are just as compatible as any other Nikon DSLR with one minor issue.

The D3000 series and D5000 series, along with the D40 and D60 are entry-level cameras. As such, they lack an in-camera focus motor. The higher-end Nikon DSLR's, basically the D70 and above, include a motor in the camera for focus.

There are two types of Nikon autofocus lenses; AF and AF-S (or AF-I).AF lenses do not have an internal focus motor, and can only autofocus with cameras that have an internal-focus motor - which the entry level cameras lack.

Nikon's newer lenses - and virtually all of the new lenses they bring to market - are all AF-S (and a few older AF-I), and have the autofocus motor built into the lens. These lenses do not require the focus motor in the camera, and will autofocus on your D3000.

Even though an AF lens will not autofocus on an entry-level camera, it is still "compatible" in the sense that you can still use the lens with the camera. It will mount on the camera, it will meter the exposure correctly, and do everything else but focus. If you have such a lens, you simply need to manually focus it.

This may be a pain for you in some situations, but realize that other situations, it is not that big of a deal. Macro use for instance - is almost always manual focused as most DSLRs do not do a good job at autofocusing at such close distances. So if you want to buy a macro lens, you could buy a manual focus or AF lens as you will be manually focusing it anyway.

Same thing goes for super wide angle lenses, such as the Tokina 11-16. Although it lacks a focus motor, the depth of field is so great that you can almost put the lens to the infinity focus mark and forget it. The only time you will need to manually focus those lenses are if you are taking photos closer than 10ft.

So really, the only time auto focus may be a significant issue to you is for normal and telephoto lenses. Whether or not that inconvenience justifies the lower cost/high performance of many of the AF lenses or not is up to you.