Memory and disk space are confused by many people and many think they are referring to the same thing. Memory and disk space on a computer are similar to humans brains and paper
Memory is the area where items are kept on a non permanent basis in the computer that it is currently using. This is similar to memory in your brain. You keep things up there that you are using in your lifetime but it is not a permanent storage. In both cases whatever is in memory when the power is turned off (in humans they die) the information in the memory is gone.
The more memory you have in a computer the faster it will work as it can work on larger items and more items at one time. Currently you will find most computers coming with 1-3 gigabytes of memory. Unless you are using a 64 bit computer 3 gigabytes is the most Windows can use. Wirth 64 bit computers you will find people often have 8 gigabyes or more of memory
The disk (or hard drive or hard disk) is where the computer permanently stores information in a format it can read. If the computer dies this information continues to be available by plugging the drive in another computer. In humans the comparable thing would be writing stuff down on paper. Since I have letters my dad wrote before he dies I can still see what he thought on stuff.
Disk drives have gotten very large for PCs. Pictures and videos take up a lot of space. Typically today disk drives on computers will range from 500 gigabytes to 2 terabytes and bigger ones are coming.
Sometimes you will run low on memory or disk space. It is important to know which so you can get the correct one increased in size