The applications they are running. Telecommunications applications include high throughput subscriber and policy databases. IT (I am referring to enterprise IT) applications normally deal with end user web-based application portals and systems. Compared to enterprise applications, telecommunications applications are built to handle higher volumes.
However, there is a convergence taking place in the telecommunications industry today that a lot of applications between telecommunications and IT are becoming more and more similar.
The term cloud is generally used in the Telco domain to refer to applications that are now run in a virtual environment and managed by the application users (or devops).