Thats variable aperture opening.. the way lens are constructed this way.. so ur max aperture opening will change while u zoom..
Meaning to say when at ur widest focal length (i.e 18 mm), ur aperture can only open up to f3.5 and as u zoom further out, ur aperture will be restricted to just f5.6 at the longest focal length (i.e.200mm)
f6.3 is 1/3 stop smaller than f5.6 so Canon lens offer more at the longest focal length
TS, bro numnumball has already explained it to you. Please do some further reading up on the basics of photography. There is a sticky titled Photography Notes For Newbies done up by Sulhan. You can start off from there.
Both the Sigma and Canon lenses are built for digital camera. DC is the acronym for Digital for Sigma lens. EF-S signals lenses for APS-C camera bodies or commonly known as 'cropped-sensor' cameras.
The main difference, besides the different aperture range (F stands for focal ratio or relative aperture), is that the Canon lens has IS (Image Stabiliser). IS is a technology where you can take up to 3 or 4 stops slower (shutter) and your image should remain sharp. IS will not work for motion blur.
IS is good when you are shooting at a slower shutter speed than the recommended '1/focal length'. E.g. If you are shooting at 100mm, your shutter speed is recommended to be about 1/125. With IS technology, you can shoot at 1/15 or even 1/8.
There's really a lot of information condensed in the name of the lens. Both manufactuerers and numerous photography review sites provide extensive converage on both lenses. Please read up on them. It will really help you more than what you can get here.