There are two types of memory: RAM and hard drive memory.
RAM: Random access memory (RAM) is used by applications when they need to store data temporarily while running. Basically, the more RAM there is on a computer system, the better its performance. Put another way, even with a high-end CPU, if you don't have adequate RAM, your computer won't be able to take advantage of its processing power and will run slowly.
How much RAM do you need? Well, it depends on what your system requires and what you are planning to do with it. That being said, a good rule of thumb is to consider 2 gigabytes (GB) of RAM as a baseline minimum. 3GB or 4GB is fairly standard to run Windows 8, and 4 GB seems to be the norm for iMac and MacBook Pro computers.
Hard drive memory: Hard drive memory is the second type. A hard drive is where all data and program files are permanently stored. Until recently, it seemed that most people took the "more is better" approach with hard drive memory capacity, but now it's more common to consider different options. For example, you may want the speed of an SSD drive and be willing to settle for a lower overall capacity, say 256GB instead of 1TB with a HDD drive. Then again, if you need a very large amount of storage for video editing purposes, you may purchase a 2TB HDD drive. In any case, 250GB is a baseline minimum for shopping purposes.
One factor to consider is the availability of external storage options, including cloud storage and backup drives. With so many convenient storage and backup options available, it's easier than ever to keep your main computer somewhat lean in terms of hard drive space. You know you can always access any files you need via backup, but you can enjoy the freedom of only keeping frequently used files handy on your computer.