CAMERA LENS HELP! Getting a 60D soon! need to know which to mount.?

I need to know the difference between Canon EF-S 18-135mm f/3.5-5.6 IS Lens, and EF-S 18-200mm f/3.5-5.6 IS Lens.

please help in providing the purpose of each lens as well as their disadvantages.

in addition, what is a 50mm f/1.8 used for?

Thanks in advances.
Update: @josh. wad about image quality between the 2 then? i've heard there are distortions for the 18-200mm.
also, does the 50mm f/1.8 perform well under low light?
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