What does a Nikon teleconverter do?

I have a Nikon D5100. I want a lens that is somewhere between 18-200mm but they don't even seem to make one. Not knowing too much about this kind of stuff with cameras? Is a teleconverter something that would help? Its just a pain to be out and to have to change lenses.
If you're just going to post rude comments go somewhere else.

Yes, Nikon does offer AF-S DX 18-200mm f/3.5-5.6

Here is the link: http://en.nikon.ca/...VR-II.html

or you can try the Tamron 18-270mm f/3.5-5.6 http://www.tamron.ca/product.htm?Product=104B008NII&Source=Category&Category=TAM_MEGAZOOM_N

or Sigma 18-200 f/3.5-6.3 http://www.sigmaphoto.com/...ized-nikon

or Sigma 18-250mm f/3.5-6.3 http://www.sigmaphoto.com/...-hsm-sigma

A teleconverter fits between camera and lens and multiplies the focal length by a factor of 1.4 or 2.

Modern types are vastly over-priced, all cause image deterioration and you lose 1 or 2 f-stops depending which you use.

The screw-on type are cheap because they're absolute cr*p, don't touch them.

A teleconverter goes between your camera and the lens and magnifies the image from the lens. You can, generally, get 1.5x and 2x converters. You just multiply the focal length of your original lens by 1.5 or 2 depending on the converter and that gives the focal length of the lens and converter. So you lose the wide angle and get more telephoto.

Nikon converters will be good but they're likely to be expensive. There are 3rd party ones, but they're generally not as good as the main manufacturer's items.

Chances are, you have the Nikon 18-55mm f/3.5-5.6 kit lens. If you mounted a 2x tele convertor on it, you'd have effectively a 36-110mm f/7.0-11 lens. With the right teleconvertor, you would have auto metering, but there isn't one made that would let you autofocus, as they all need a lens at least as fast as f/2.8 to do that. A tele convertor isn't going to be a magic answer here.

As for the 18-200mm, I've had Nikon 18-200mm AF-S VR since 2007. My brother got a Sigma 18-200mm for his camera, and you'll find similar products from Tamron.

One big factor for your decision is the need for a built-in motor for autofocusing (BIM). Here's a webpage that lists all the lenses that autofocus on your D5100.
http://en.wikipedia.org/...cus_motors

And, please don't take this as rude, but one of the reasons for getting a dslr is the ability to change lenses. I can change my Nikon lenses, pull one out of the bag, and replace the other one within 5 seconds. Even though I have the Nikon 18-200mm AF-S VR lens, I sometimes leave it behind because 2 or 3 other lenses do the job better in a situation. For travel, the 18-200mm is great, but for low-light, portrait, action, and sports photography, it's limited.

A teleconverter fits between camera and lens and multiplies the focal length by a factor of 1.4 or 2.
and yes, nikon has a 18-200mm
http://www.amazon.com/...002JCSV8A/
about 900USD

or go get a tamron 18-270mm and it works on yr nikon