A LOT of people get the terms confused.
Memory is little strips of chips that are in slots on the mother (main) board inside the computer. Memory is not the hard drive.
Hard drive is where you see Gigabytes. Hard drives are for file storage. If you open one up, it looks like a miniature record player, with layers of CD looking discs and a little arm that goes across each one. Just don't open a good drive. Keep the autopsies for dead drives.
If it starts making loud clackety clack noises like quarters in a clothes dryer sound, better start backing up your important stuff, because that's a signal the hard drive may crash not too far in the future.
RAM stands for "Random Access Memory." It's like the traffic cop for the processor (brain). Information goes through the RAM before it hits the processor. If a computer is a bit low on memory, it may access the hard drive for temp storage until there's enough room on the "highways" to get to the memory.
The amount of total RAM is determined by the type of motherboard you have and what amount of RAM came with the computer. It will have listed a max capacity and type for RAM in the booklet. There are different types of memory sticks.
Inside the computer is also a ROM, Read Only Memory. This is usually supported with a large battery that looks like a watch battery. It stores the beginning steps of instructions for the computer to use when it first starts up. Don't take out this battery unless it's dead, which I've only seen happen once with a computer.