Memory, in computing (let alone in the general non-technical sense of the term), is a diffuse and complicated thing to explain. It is also something that programmers can treat as an abstraction, because the way a piece of code treats memory, the memory could be microscopic pits on a piece of wax or DNA in a bee's wing. For those of us on the hardware side of computing, memory is a bit easier to explain: it is that stuff that comes in little modules (not that I've ever heard anyone ever say "modules", sticks is the preferred vernacular), and it is the stuff that goes in memory slots.
Long ago, in the dark ages of computing, around pre-1990 or so, memory was inserted in chips directly onto the motherboard. This worked fine for most people's needs, but as computing crossed over into the general populace, and more customers wanted to customize and upgrade their computers, motherboards were designed for various ways to switch out and increase the amount of memory. These took the form of various small chips that could be inserted into, and removed from the motherboard with no special tool. The first was called the Dual In-Line Pin Package, but it was soon replaced with various types of inline memory modules:
- 30-pin SIMM: 386-486 era
- 72-pin SIMM: 486-Pentium era
- 168-pin DIMM: Pentium-Pentium III era
- RAMBUS: Pentium III era, did not become widespread
- DDR and DDR-2 Memory: The current Standard
- (There were also laptop variants of the above mentioned, which are smaller)
Although the technology has increased greatly since the first SIMMs (with a capacity of around a megabyte to the current day (with a capacity in the Gigabyte range, the physical appearence of memory modules has remained very similar. All of these memory modules are a few inches long, with a line of connectors, usually made of gold, where they connect into the motherboard. They are made out of green PCB, (although occasionally, as with other computing parts, you see someone try to fancy things up with colored circuit board), and have a number of small black chips, on either one or both sides. They are usually a bit under an inch tall, and about an eighth of an inch thick. A motherboard usually has from two to four slots for memory modules, although some desktop computers have more, and servers can have dozens. Removing and inserting a memory module can usually be done in a matter of literally seconds. Although they are not specifically fragile, having no moving parts, it is a good idea to keep them away from electricity, corrosive chemicals, fire, and sudden shocks, or anything else you wouldn't want to expose yourself to.
There is of course, many more technicalities to memory modules than this, involving such fun subjects as clock latency and voltages, but in general, the easy packaging of memory into modules is something that has made computer memory very simple, at least from the hardware side.