The document discusses different types of multithreading in operating systems. It describes multithreading as allowing an application to divide tasks into individual threads that can be performed concurrently. The types discussed are many-to-many, many-to-one, and one-to-one relationships between user threads and kernel threads. In many-to-many, there are multiple user and kernel threads. In many-to-one, multiple user threads map to a single kernel thread. In one-to-one, each user thread maps to its own kernel thread.