What is pagefile or swap file ?

Basically there are two types of memory used by operating system. In general they are known as physical memory and the virtual memory. More specifically RAM(Random Access Memory) is the type of physical memory and virtual memory generally the storage medium such as harddisk, flash drives etc. 

But what is Pagefile?

When RAM or physical memory is full then it stores some portions of its data that recently not used, in some portion of harddisk, or other storage that specified as the extension of the RAM, is known as pagefile. Simply some portion of storage medium are specified as the extension of the RAM where RAM store it's recently unused data, is known as pagefile. 

Pagefile is a common name for Windows operating system, and in other operating system this are known as swap file. Pagefile or swap file are the same thing.

So a pagefile or swap file can be simply defined as space in hard disk or other storage medium that is used as virtual memory.So organize your pagefile allotment for better optimization of your PC performance. You can also add some flash device as virtual memory to increase your PC performance.