On the zoom lenses that you mention, 24-70 and the 75-300, smaller the number the more of a scene you can get in, but the smaller the stuff in the picture is. So if you are in a small room and you want to photograph everyone sitting around unwrapping presents or whatever then you'll be using the 28 end of the zoom, to get the same level of zoom on an XT you need a minimum zoom focal length of 17 or 18. When you want to focus in on one item and make it seem much closer and larger than it really is then you'll use the 300mm end of the focal range.
same scene, but different lenses
40 mm
300 mm
The other numbers, the ones behind an f, like f4-5.6 or f1.8, are the aperture size. Just like the pupil in your eye each lens has a diaphram that can be large or small to let in less or more light. The smaller the number the larger the aperture can get. The larger the aperture the more light gets in. The more light gets in the faster shutter speed you can use.
With faster shutter speeds and larger apertures you can more easily take pics in low light or action shots. Another "effect" that the aperture affects is the depth of field. If you are photographing a subject, car, family member, flower, etc... and you want only the subject to be in focus, but the background to be blurred so that the subject really stands out you need to use a large aperture. If you want to get in lots of foreground and background then you want to use a small aperture.
large aperture/small f number
small aperture/large f number
A lens with a maximum f-stop of 1.8 opens up large to let a lot of light through, that means that you can get enough light to the film/sensor in a much shorter period than if the lens was f4. On a zoom lens you'll see something like f4-5.6 because the f-stop changes as you zoom the lens through it's range.