Virtual Memory: A computer can address more memory than the amount physically installed on the system. This extra memory is actually called virtual memory and it is a section of a hard disk that’s set up to emulate the computer’s RAM. The main visible advantage of this scheme is that programs can be larger than physical memory. Virtual memory serves two purposes. First, it allows us to extend the use of physical memory by using a disk. Second, it allows us to have memory protection, because each virtual address is translated to a physical address.
Thrashing: Thrashing is a condition or a situation when the system is spending a major portion of its time in servicing the page faults, but the actual processing done is very negligible. High degree of multiprogramming(if number of processes keeps on increasing in the memory), lack of frames (if a process is allocated too few frames, then there will be too many and too frequent page faults) causes Thrashing.
Threads: A thread is a single sequential flow of execution of tasks of a process so it is also known as thread of execution or thread of control.