The difference is that there is no such thing as secondary memory. It's either System Memory, or it's Hard Drive Space. Each one does something COMPLETELY different. People just tend to call the Hard Drive Memory because it "Remembers" Information. In any event, the hard drive is NEVER Memory. If you're lacking enough actual memory to do the job, the hard drive can sometimes host space for a page file. it allows the system to write to a pre-allocated space when it runs out of actual memory for a temporary amount of time. since the hard drive is slower than memory, this really doesn't help performance all that much. it's better to go out an buy more memory when you see alot of hard drive paging. Let me list some of the things that memory and Hard Drives so different.
Hard Drive:
1 Will have ALOT more capacity. Up to 1 Terabyte
2 Is SLOW compared to System Memory
3 It has Moving Parts
4 It is sensitive to Movement and Shock
5 Is NOT required for your computer to boot.
6 Information is Retained when the computer is Turned off.
Memory:
1 Usually comes in DIMMs (Dual Inline Memory Module) that are 2GB or Less
2 Is alot faster than a Hard Drive
3 Has NO MOVING PARTS
4 Is Unaffected by movement or shock
5 Is required for a successful Boot.
6 does not store information when the computer is off.
A hard drive is nothing more than storage. it's like a closet. Memory is like scratch paper for your CPU. It's a Buffer really. Your CPU is faster than your Hard Drive is, so there are alot of programs that will have information PreFetched into memory. this way the CPU will have it right when it's needed rather than having to wait for the Hard Drive to find that information. In the future, long term storage will all be in the form of actual memory, but right now it's too expensive to implement. Hard Drives are the last major bottle neck of computer performance. the next time someone says something about primary and secondary memory, you have my permission to call them an Idiot.