CtR is this containerized install-from-web platform that Microsoft came up with for .NET applications. Yeah, it's actually a lot of years old. But generally speaking, if you get an Office disc or ISO file (say, from Microsoft's Volume Licensing Center), and install it, you get a traditional Office install that gets serviced via KB downloads from Windows Update. If you get Office via a web download from office.com, you get CtR, which self-updates itself sort of like modern web browsers.
A huge difference for business is that Windows Update allows you to approve/deny updates and cache them on a local server with WSUS, whereas CtR leaves each machine to go talk with Microsoft amongst itself on what to install and mostly cuts your IT staff out of the loop.
I've never been clear on what CtR describes. The first CtR Office was Home & Student 2010, which was a purchase.