The XT stands for EXtended Technology and was introduced in early 1983. It is basically an improvement over the original PC but not really that much different in that you could upgrade a PC to XT specifications with a few changes on the motherboard. As standard equipment came the tried and true 8088/4.77mhz processor from the PC, 128k RAM (Upgradeable to 256k), 5.25" Floppy Drive (Full-Height), and a large (for the time) 10-mb Full-Height Hard Disk Drive.
There is also 2 versions of the XT MotherBoard, both released at different times. The Original was a 64/256k Max RAM on board whereas an updated Motherboard with support for the 101-key keyboard, 3.5" Floppy Drive and a few other modern niceties were included on the 256k/640k RAM motherboard released on 1986 BIOS date XT's. I happen to have one of the more modern XT's and find that having 640k of RAM makes it reasonably useful for running word processing and a basic text-based internet browser.
Since I do have one of these, I do know a little more about them and also will be scanning the machine itself to give you an idea of what is inside. These machines ARE NOT light by any standards. Considering what little they do, they seem really heavy. But this also makes them bullet proof when it comes to durability. Much like older cars the same can be said about computers "They don't make 'em like they used to". It applies in both a good way and bad way. The good thing is that older pc's usually always have something wrong with them (HDD's are usually the most annoying thing that fail), but the fact is that the motherboard itself is pretty much indestructible, no heatsinks or fans required. On a modern Pentium PC running Windows 95, you get so many GPF's they become second nature to you. Those of you that run DOS, UNIX, and other text based OS's, how many times can you remember your pc locking up on you. I can bet alot less often than Windows 95 does.
Since the XT is in the middle of the original PC's, it is also in the middle of the pack as far as performance goes. A 5150 PC will not do very much in stock form other than boot up and run DOS. This is in contrast to an XT, which is really the earliest PC type computer able to run the Internet reasonably well. I use a 14.4 modem and find the speed to be excellent considering the age of the machine and the slowness of the HDD.
As I said before certain later models of the XT will be compatible with a 3.5" drive and 101-Key keyboard. I think the easiest way to tell the difference is to look at the motherboard which should have printed on it either 64/256k or 256k/640k. The latter is the more modern one. Of course if you would like to upgrade your older XT to 640k, it can be done by performing a little upgrade of your own. A link to the file is right here.