Computer memory, also called RAM (Random Access Memory) is the short term memory for a computer or electronic device. When a program is loaded, information is taken from the hard drive and placed into the computer's memory so that it is ready for the computer's processor to receive and do something with.

Most computers can accept between 2-4 "sticks" of RAM.  They're sometimes referred to as sticks because their shape is similar to a stick of gum: long, flat, and thin. Each stick of RAM has a different, and ever increasing, measurement of how much memory it can store.  It wasn't too long ago that RAM could only hold a few MB, but today, it's common for RAM to hold several GB. 

Regardless of the specific amount of memory each stick can hold, it is almost always less than the total capacity of the computer's hard drive, and more than the memory connected directly to the computer's processor (often referred as the CPU cache).  It's also far faster than the typical hard drive, but slower than the CPU cache. 

When the computer is turned off, all of the data stored inside RAM goes away, which is why computers have hard drives - for permanent storage even though they're many times slower than the computer's memory.

Often, the quickest and cheapest way to increase the overall speed of a computer is to add more RAM.  If the collective programs loaded and actively running on a computer use up more memory than what is available, the computer is forced to use the hard drive to compensate.  Since the hard drive is many times slower, this slows everything down; therefore, making sure the computer has enough memory is essential.  This only applies to programs actively running.  If a program is simply installed, but not running, it is not loaded into memory and therefore does not affect the amount of RAM needed. When the amount of physical RAM is exceeded and the computer is forced to use the hard drive, it places the data in a special file called a "swap file".

Sometimes programs have what is known as a memory leak.  Internet Explorer and Firefox are notorious for various memory leaks. On Windows, check your task manager or on Linux and Macs, check your system monitor, top, or ps to see which programs are loaded into RAM.  If the amount of RAM is much higher than when you first loaded the program, chances are this is the case.  Simply restarting the program can be a temporary fix especially if it takes several hours to notice the slowdown. In these cases, make sure you have the latest version, in case the issue has been fixed and make sure your software vendor is aware of the problem.