Both terms are used to refer to internal storage space on a computer. Memory, usually referred to as Random Access Memory (RAM), is the place where an application loads its data during processing, while a hard disk drive (HDD) is usually the place where data is stored for long or short term retention. The line blurs at paging (otherwise known as swap space), where a portion of the HDD is turned into memory for the computer to swap out applications and data from memory to the disk. In this way, the HDD is made into memory, which, while slower than the RAM in the system, can be usefully used to create larger virtual memory spaces for systems. Most systems will create a swap space (paging space) that is equal to two times the real RAM space on a computer. Also, system memory, or RAM, is usually more expensive on a per megabyte (MB) basis than, say, a MB or gigabyte (GB) of hard disk storage space.
I like to think of the computer's memory as a washing machine, the place where you actually do the washing and the work gets done, versus the water storage container, which is like the computer's disk drive/storage medium, where the water gets held waiting to do the work in the washing machine. This metaphor may help you to keep in mind the difference, since they are often confused, mostly in the specific amounts of each that are in your computer system.
In a computer system, you will also find another kind of memory, called Read Only Memory (ROM), in which the system keeps file that are to only be read, like system firmware or BIOS. You can flash this kind of memory and load updated information. There are a host of other types of memory that act like HDDs for storing everything from MP3 and pictures, to presentations and other data. You may have seen these in a variety of formats, like USB keychains, CF cards, SANDisks, or memory sticks, but they all serve basically the same purpose, storing data, not applications data while they are running like system memory-RAM does. One other important difference to consider is the state of the data when the system is turned off. In the case of RAM, the data is lost, which is why you shut down major OS platforms, to flush the state of the memory and the application to disk, which retains the data when the system is turned off.
I hope that the above helps you with your question. Let me know if you have any questions or concerns. Thanks!
This was first published in August 2004