USD is the Universal Scene Description that pixar coined a few years ago. it wants to be gltf but more for exchanging files between graphics packages. The general idea is that many graphics packages do things a tiny bit different and that means every package has basically his own file format. So the idea for this format is to store anything you need to open a 3d scene and work on it in the file. So not only all the models and textures are there such as a fbx or gltf file would have, but also all the bones, animations, lights and light settings are in the file.

Because the files can become really big in the graphics world and because it one more standard on the pile of standard developers often use different names for USD. They might add the letter p in front for Pixar. but the most common is to use the USDz extension basicly the file is now a zip file just like a jar is.