The hard drive is the main storage component on your computer. Every photograph you've saved, every song in your iTunes library, is taking up space on your hard drive. A removable piece of hardware inside every machine, the hard drive stores not only any data you create, but also the operating system of the computer as well.
This storage space is def. something to consider when purchasing a new computer, or upgrading your current set-up. Average hard drive space these days, on a brand new machine, will most likely be between 250-500 gigabytes (GB.) Machines a few years older, probably have between 40-120 GB drives. For comparison, the average digital photo is just over 1 megabyte (MB,) and 1,000 MBs equal 1 GB.
As your hard drive is used over time you may need extra storage, or want to back up your data. External memory can help with either of these. Flash drives, or thumb drives, are usually small capacity USB devices that can be used to store several files on and are extremely portable. Most are able to be attached to a keychain. While it's possible to get higher capacities, most flash drives now hold up to 8GB.
External desktop drives connect the same way, but are physically larger and have greater storage capacities. They require that their own power source be plugged in, and are usually meant to stay in place. External portable drives complete the same function, with high capacities, but don't require a power source. They're usually lighter and smaller, giving them the portability of a flash drive, with the storage capacity of a desktop drive.
Knowing your storage needs will help when choosing the right sized computer or external drive. Definitely bring up these concerns when purchasing or upgrading a machine.