RAM is perhaps the most important of the input/output devices. When we talk about computer memory, we are mainly talking about RAM. In this class, when we think about the banks of light switches that can be manipulated, we are thinking of RAM memory. The term Random Access is pretty unfortunate. There is nothing random about how memory is accessed. The program running will determine what is in memory. (of course, the program itself is in memory too!) RAM can be read from by the CPU. This means that the CPU can 'look' at any address in RAM and get the contents of that address. It can also be written to by the CPU, meaning that the CPU can change the value of memory cells on the fly.
One VERY important aspect of RAM memory is that it requires power. RAM can only hold values while power is going through it. If the power is interrupted, the RAM will lose ALL the values in it. This is why it is so important to save your work frequently when working on a computer. RAM memory is volatile. When the room you are working on is hit by a tsunami and the power goes out of your computer, you lose everything that was in RAM. This could be bad. The answer is to make copies of RAM, and place these copies on other kinds of media. That's what disk drives are for. (see below...)
The amount of RAM in your computer is obviously a pretty important factor. The more memory you have, the more 'room' there is in your computer for information and programs. Modern programs have gotten HUGE, and the kinds of information you can work with have gotten much larger. Some early home computers had 4 or 16 K of RAM. The original IBM PC had 640 K of RAM. In its day, this was thought of as an extravagant amount of memory that would never be fully used. Modern computers with less than 16 Megabytes (16,000 K) are considered a bit lame. (pun completely intended.) Older computers can be quite happy with much less RAM, but they will not be able to run the newer programs.
If you are buying or upgrading a computer, you should seriously consider as much RAM as you can afford. There is probably no more cost-effective upgrade than RAM.
If you find that you need more memory, It is relatively cheap and easy to do a memory upgrade. Note that not all types of RAM are interchangeable. Look in the book that came with your computer for specifically which type of RAM you need when you are ready to upgrade.
Most of the time when we speak of memory, we are only thinking of RAM. In computer advertising, RAM is sometimes referred to as DRAM for Dynamic RAM