Computer RAM is the best known form of memory your computer uses. Every file or application opened is placed in RAM. Any information the computer needs or uses becomes part of a continuous cycle where the CPU requests data from RAM, processes it and then writes new data back to RAM. This can happen millions of times a second. However, this is usually just for temporary file storage, so unless the data is saved somewhere, it is deleted when the files or applications are closed.
A Hard Drive is a form of computer memory that allows you to permanently store data. This is where all of your permanent files and programs are stored. On computers running with Microsoft windows the Hard Drive is often called C-Drive. The size of a Hard Drive is typically measured in gigabytes.
Virtual memory typically comes into place when applications are too large for the RAM to handle. The operating System uses the hard drive to temporarily store information and take it back when needed. This is normally a lot slower than actual RAM and can possibly degrade performance if used to heavily.
Cache Memory is used in-between the CPU and the RAM and holds the most frequently used data or instructions to be processed. There are three different grades of Cache. Some systems will only have level 1 and level 2. More advanced systems will include the level 3.