I'm wondering what's the difference between a standard lens and a lens that can zoom from close to far...
But let's not discuss the obvious: zoom, price, weight.
Is there any picture quality difference between the lenses? Will the 18-250mm lens have slightly "dark edges" and straight lines distorted (like bulbing/curving) when not zoomed afar?
Listing brands, at the very least the brand of your camera, is useful.
In any case, here are links to some reviews that pick out the less-than-obvious features of 18-70 and 18-250mm lenses:
Nikon 18-70mm
Photozone
SLRGear
Sony 18-70mm
DPReview
Photozone
SLRGear
Tamron 18-250mm
Photozone
SLRGear
Sigma 18-250mm
SLRGear
In a nutshell, the Nikkor is a solid performer, the Sony is not (corner softness and high CA throughout range), the Tamron superzoom is surprisingly good considering its range but still has the limits of a superzoom, and the Sigma's sharpness suffers significantly in its telephoto focal lengths.
Compared to the Nikkor (not so much the Sony, though it's still better optically than the superzooms), you'll compromise focus speed and max aperture (i.e. max at 70mm on the Nikkor is f/4.5 whereas it's about f/5 for the others) when using the superzooms, not to mention optical quality (sharpness, CAs, distortion, etc). Also, the superzooms are prone to lens creep.
In general, the superzooms trade off optical quality and aperture for their wide range, although there isn't a *huge* difference in quality until you compare them to primes or f/2.8 lenses.
Note that Canon does not make lenses in these specific ranges.