Tell me pros and cons of nikon d5100 and d3100. More emphasis on d5100?

Also tell me if nikon plans to launch any new camera this year close or a little costly than these two cams?
> what does the absence of focus motor mean? How is it bad and how a d5100 buyer would deal with it?
> what is the accessory Ferrite core? What's its use?

The D5100 is a fairly new Nikon camera and probably won't be replaced this year. It's gotten some pretty amazing reviews and is definitely a well-designed camera for the novice market.
http://www.kenrockwell.com/.../d5100.htm
http://www.dpreview.com/...ikond5100/

The entry-level Nikons don't have an autofocus motor in the body, but rely on an autofocus motor in the lens. Therefore, they're most compatible with Nikon AF-S lenses, but will manually focus with all Nikon AF and AF-D optics. Before that worries you, be aware that EVERY Canon DSLR is in the same boat. No built-in af motor and requiring an af motor in their EF and EF-S lenses.
Here's a list of the many lenses (growing every month) of full-compatible lenses.
http://en.wikipedia.org/...cus_motors

The Ferrite Cores are attached to cables to suppress high frequency noise levels in electronic signals. I've had them on other connecting cables for different devices.