Hi Mike I'll answer the bits i know and let some else cover the rest.
AF= autofocus, meaning when you half press the shutter button the camera will try to lock focus on the subject over the focus point(s) you have selected.
IS= Image Stabilisation, typically in two modes, one counteracts movement in the horizontal plane and one counteracts movement in the vertical plane. By movement i mean camera shake. In theory IS means that you can shoot a subject using a lower shutter speed than with a none IS lens.
F/ Stop. this is a ratio of the area of the opening of the lens (apperture) divided by the focal length.
A) so you could have a lens with a focal length of 280mm and an area of 100mm Sq for your apperture this would give you an F-Stop of f/2.8
B) you might have another lens of 280mm focal length but the area of the opening is only 50mm Sq, this would mean that half the amount of light is let into the lens and would give you an F-Stop of f/5.6
so lens A is one stop faster than lens B, this difference is crucial for photographing subjects in low light.
Ideally you want a lens with 1) a constant apperture (pricey for a zoom lens) 2) an apperture that is as wide as possible
typically the lens that are faster will be more expensive than other lens of the same focal length as an example the Canon 400mm f/2.8 which is a standard lens for football photography now retails at over £6,000. the 400mm f/5.6 retails at less than a quarter of that price.
I hope some of this makes sense, but i've just got up and my brain ain't awake fully.
For buying lens, first i would think carefully about the areas you want to photograph. Sports and wildlife typically mean lens with a long focal length and they get pricey VERY quickly. Landscapes usually require shorter lens at a guess anything from 35mm down and portrait probably 50mm to 200mm.
You should buy the best you can afford and if you invest in high quality well built glass, it will last you years (providing you look after it)
Thanks
Andy
