What is memory ?

The term "memory" is sometimes used to describe hard disk space and RAM in a computer. These things are quite different as explained below.

Hard Disk is long-term (archive) storage

Your computer might have 500GB of hard disk space.
Data is stored as patterns of magnetised areas on a flat coated glass disc.
Your computer's hard disk stores the operating system (e.g. Windows ™), your settings and your files. These files are maintained on the hard disk even when you turn the computer off. To edit a file (e.g. add some words to a paragraph, change the contrast in an image etc), the computer loads a copy of the file into RAM. It does this as it can work much more rapidly with the data in it's memory. Changes are only committed to disk when you save the file.

RAM (memory) is short-term (working) storage

Your computer might have 4GB of RAM free.
Data is stored by tiny electrical charges in microscopic structures inside a computer chip.
Data in RAM can be read and written to thousands of times faster than to the hard disk, so this is where files, or parts of files are loaded to work on. Data would be lost if the power was turned off before you saved the file. You can often make your computer appear to run faster by adding memory.