One topic you might hear people discussing when they're talking shop about computers is how much random access memory (RAM) they need to add to their computer. Up to a point, adding RAM will normally cause your computer to seem faster on certain types of operations. RAM is important because it eliminates the need to "swap" programs in and out.
When you run a program such as a word processor or an Internet browser, the microprocessor in your computer pulls the executable file (.exe) off the hard disk and loads it into RAM. Large programs like Microsoft Word or Excel use large amounts of memory. The microprocessor also pulls in a number of shared dynamic link libraries (DLLs) -- shared pieces of code used by multiple applications. The DLLs take many more megabytes.
Then the microprocessor loads in the data files at which you want to look, which might total several megabytes if you are looking at more than one document or browsing a page with a lot of graphics. So a big application can easily take 100 megabytes of RAM or more, which can slow your system down significantly if there isn't enough memory. On your machine, at any given time you might have the following applications running:
Besides all of those applications, the operating system itself is taking up a good bit of space. Everything together may need more RAM than your machine has. Where does all the extra RAM space come from?