A common question I get is "I have a lot of photos/music stored in my PC - won't that make it run more slowly?"
A brief answer is "No" and the reason is as follows. (Caution - some jargon included and hopefully explained without being too technical).
The photos or music are stored in your Hard Disk Drive (HDD) inside the System Unit (the big metal box with the power switch and CD/DVD drive). A typical HDD has a size of 100GB to 250GB.
[GB = GigaByte = 1000 million characters. An A4 page of text uses about 10,000 characters. So 1GB is equivalent to about 100,000 pages of text! ]
The working area of your computer is its memory [jargon term RAM = Random Access Memory]. The typical memory size of a new PC today is 1 to 3 GB. Not all of this memory is available to use for your work as the system needs to run a number of background tasks to keep itself running (just like your brain is automatically controlling your breathing and heart rate without you having to think about them).
Think of the memory as being like your table where you are working. There is only a certain amount of room to lay out your papers without them becoming cluttered and hard to find. Equally, you can only have a limited number of programs running before the computer starts to slow down.
Think of the HDD as a filing cabinet. You can store thousands of sheets of paper in a filing cabinet, but as long as they stay in the filing cabinet and not on your table top then your table top stays uncluttered and you can get on with your work.
The confusion often arises because users think of the HDD as "memory" because it has to remember the contents of your files. Better to think of the HDD as storage and save the word " memory" for the RAM.