I just want to know the actual difference between android dev phone, android nomral phone and android rooted phone.
Can we not buy dev phone from a physical shop or from some where else except android marketplace?
Can we buy android rooted phone? If yes then from where ?
A developer phone can only be bought from Google, if you are a registered developer. The Android source tree contains (almost) all source code needed to re-build and flash the phone.
A normal phone is what you can buy in a shop.
A rooted phone is a normal phone that someone has gained root access to. Root access does not necessarily mean that you can re-build the software, although there are some community-driven projects that will help you do that, for example CyanogenMod.
You can't buy a rooted phone, instead you buy a "normal" phone and root it yourself. Have a look at XDA Developer's forum for some guidelines.
The one exception is the Samsung Nexus S that you can buy as a "normal" phone, unlock the boot loader and then build software for using the Android open source project. I assume the Galaxy Nexus will give you the same possibility, but I haven't tried it myself yet.
If you are developing apps, you do not need root access. You can just use any normal Android phone for that. But if you are developing some platform features, you need a rooted phone or a developer phone. Or you can get one of the many developer boards available, like the Beagleboard and the Pandaboards. These boards will not look like a phone, but they are superb for platform development.
Basically, it all boils down to what you are going to use it for.