Helping people with computers... one answer at a time.
The difference between the various types of things we might call "memory" is a fundamental concept that's often confused. I'll review the basics.
Memory, hard disk, and RAM: I get a surprising number of questions that show a misunderstanding between these most basic of computer terms.
The most common mix-up is that people believe they are the same. While they're not, I can see how it could be easy to confuse them at a conceptual level. So, let me explain these three terms.
RAM: Random Access Memory
RAM stands for Random Access Memory. RAM is a collection of electronic circuitry where your computer's programs and data are stored when it is running.
As you can see to the right, RAM is one or more small circuit boards containing several integrated circuits where the actual microscopic circuitry lives. Each computer typically comes with a certain amount of RAM installed. It also has a limit to how much total RAM it can contain.
The machine I'm using as I type originally came with two gigabytes (roughly two billion bytes) of RAM, and has since been upgraded to its maximum capacity of eight gigabytes. This maximum is a hardware limitation of the main computer circuit board or "mother board" into which the RAM modules are inserted. If I wanted to exceed this limit, this computer would need a new motherboard, although most folks would simply get a new computer with greater capacity.
The one characteristic that differentiates RAM from other types of data storage is that it requires power to retain its contents. Remove power from RAM and it forgets everything that it had contained.
An Opened Hard Disk
A Hard Disk Drive (HDD), or sometimes just hard disk or hard drive, uses physical platters coated with magnetic material to store data. As the disk spins underneath read/write heads, the changes in magnetic polarization can be read by the read head, or they can be set or written by the write head.
A hard disk is a physical device that typically resides in your computer, although it's very common to have external hard drives connected to your machine using USB or Firewire.
There are two things that distinguish hard drives from other forms of storage:
A hard drive involves moving parts. The platters spin at speeds measured in thousands of revolutions per minute and the read/write head moves back and forth across the spinning platters.
Because magnetic and not electronic components are used for storage, a hard drive retains its contents even when the power is removed.
Computers come with a hard disk on which the operating system and initial set of programs are installed. This is where you'll save your work. You can replace the hard disk with a larger one, taking care to move the data from the old hard disk to the new. You can also add additional hard disks either internally, if your computer has room, or externally, through USB and similar interfaces. Technically, there's no limit to how much hard disk space most computers can have, but practically, the sheer number of drives, connections and power required for more drives limits the amount of space you can add.
The machine that I'm working on now came with a 300-gigabyte drive. It has since been upgraded to include a one terabyte (one trillion bytes) drive, a 1.5 terabyte drive internally and a 500-gigabyte drive attached via a USB connection.
Memory is RAM. It's that simple.
Normally, the term memory refers only to RAM, not your hard disk.
It's this confusion that I see frequently. A statement like, "I just added 500 gigabytes of memory to my machine," is unlikely to be true. You might have added 500 gigabytes of hard disk space, but at this writing, 500 gigabytes of RAM is well beyond the reach of any consumer PCs, which tend to max out in the eight, 16 or maybe 32 gigabyte range.
Memory = RAM.
Hard disk space = hard disk space.
I can see why there's confusion. As hard disks are used to save and "remember" stuff, this certainly sounds like memory. But that's simply not how memory is used when it comes to computers. Memory means RAM.
A USB memory stick
Newer technologies blur the lines of which technology is used where.
Flash memory, for example, is used to make USB Thumb drives. Flash memory is a type of circuitry that's similar to RAM, except that it retains its contents when power is removed . Hence, it's most frequently used to create what we've come to call USB sticks, thumbdrives, jump drives, RAM sticks and a whole collection of other terminology.
Circuitry, sort of like RAM, is used to make storage that's sort of like a hard drive.
The important take-away here is that flash devices, even though they use flash memory, are not referred to as simply "memory". If anything, they're simply called drives, or flash drives. Occasionally, they might be called flash memory but it's the "flash" that distinguishes this from the plain old "memory", which is RAM.