Nikon D5100 18-105mm or 18-55 & 55-200?

Is there much of a difference between 105 and 200 mm lens? Because I don't want to buy two lenses if I don't have to.

Yes, huge.

If you don't want to buy two lenses and swap them around, get the Nikon 18-300mm.

Yeah! I agree with Keerock. Doesn't your D5100 comes with a kit lens? If it does, then just upgrade your lens starting from 55- 200/300, but if it doesn't and you're short of money, stick with the 18-105 would be okay, anything starting from 55-200 is considered as telephoto lens which makes you more harder to focus on things you'll be needed to capture everyday like: food, people, and so on. You know people would love to have long/huge lens on their camera, it shows that you're likely a professional, so many people will choose the longer focus length lenses, well simply just a poser haha! All the best!

Yes, I use the 18-55mm VR as an everyday carry around lens. The 18-105 is even better. The 55-200 is a good lens, but it's better suited for taking photos of things at a greater distance.
I have the 18-55mm & 55-200mm lens myself.

Not a huge difference. I've got the 18-105 and it is nice to have that little extra reach over the 18-55 (though not super necessary). I will usually leave my 70-300mm in my bag or at home unless I know I'm going to be shooting wildlife, aircraft, or sports. It takes lots of mm at the long end to make a difference, as the difference between 200 and 300mm is nothing to write home about.