I’m more familiar with Nikon, but from what I can remember, yes, all new lenses will work with your XT. The mount you specifically take is the EF-S, but EF lenses will work just as well. The thing to be wary of is EF-S lenses you might buy now, won’t work on a full frame camera should you upgrade to a pro or semi-pro body.
All cameras have a sensor in them it’s what makes them digital and it’s what records the light. The lens directs the light onto the sensor the sensor translates the light into RGB pixels. Most consumer level D-SLRs have what’s known as a “crop sensor” while professional level D-SLRs have a full frame sensor. Full frame sensors are bigger, and are roughly the same size as the traditional 35mm film. Crop sensors are smaller. I’m not too sure on the actual science of it all, but the general difference is in the amount of pixels that are packed into a given area. Allowing for a bigger surface area for the pixels and to spread them out means you get better quality, so professionals will go for full frame sensors, most DPReview information charts give a rating on pixel density (this could be all wrong, but it’s the idiots guide to it that’s been told to me many times.) The bigger sensor is of course more difficult to make, which is partly why pro-bodies are so much more expensive.
When it comes to lenses, there can be two general types. One type for full frame sensors and one for crop sensors. Crop lenses won’t work on full frame sensors. Because the crop sensor is smaller, it means the crop lens has to redirect the light to smaller area, this makes them easier and cheaper to make, but it also means that if they were put on a full frame sensor, the light wouldn’t extend the full way around the sensor. However, because full frame lenses direct a greater area of light onto the crop sensor, it’s fine to use with the smaller sensor. However the trade off is that because they’re directing a greater area of light onto a smaller surface, some of the “image” gets cut off. It’s not recorded in less detail by the sensor, it’s just the sensor is recording a smaller area than what the lens can actually send, hence “crop.” Following from that a lens that’s rated at 300mm, actually becomes in effective a 450mm(480mm on Canon because they’re 1.6 crop factor) lens on a crop body, because the smaller sensor has the effect of zooming in on a smaller area. It records it in no less detail, because the light isn’t actually made of pixels, it just means that you get a greater telefocal ability than the lens would give on a full frame sensor. This can work out for you if you typically demand greater reach with your lenses. But conversely, if you prefer wide angle shots it means where say you would use a 17mm lens on a full frame sensor body, you now need a 12mm lens on the crop body. This all means that if you see yourself upgrading to a professional level camera in the future (within ten years or so) any saving you make on EF-S lenses might be a bad thing because you’d have to dump/replace them all when you upgrade to the full frame body. I wouldn’t worry about it though, unless you’re already eyeing up a full frame body or have plans to get into photography at a professional level.
But yeah, all in all, any new lens you buy with a Canon mount should work your camera, even if it’s not made by Canon. If you’re looking into second hand lenses, most recent ones should work but you should be careful in checking if autofocus technology and the like makes a difference with off-brand lenses. If you’re looking at lenses 15+ years old, you probably need to check all the options, even if you’re buying Canon brand lenses.
