"Storage capacity" refers to the total size of your computer's hard drive. The storage capacity of a drive will be measured in megabytes, gigabytes or terabytes (depending on its overall size). 1024 megabytes equals one gigabyte, and 1024 gigabytes equals one terabyte. If you want to increase the storage capacity of your computer for the purposes of storing more files, you have three different options.
Buy a USB flash drive. This will be the cheapest method you can use to increase computer storage capacity. USB flash drives are small, portable hard drives designed to fit on your key-chain or in your pocket. They plug right into a USB port and are ready to go. You can find USB flash drives as small as 128 MB or as large as 100+ GB, with new models with higher capacity being released every day.
Buy an internal hard drive. This method will require a little bit of technical know-how on your part, or at least a willingness to pay a professional to perform the installation. Most desktop computers (and even some laptops) come with empty space inside their case to be used for expansion hardware. This includes space for hard drives. You can put an internal hard drive inside your computer and instantly increase your storage capacity without changing your machine's appearance.
Buy an external hard drive. External hard drives are like USB flash drives, only they are much larger and as a result can store more files. External hard drives have been released that can store over 2 terabytes of information on a single drive. An external hard drive will sit comfortably on your computer desk and plug into your machine via either a USB cable or a FireWire cable (whichever you happen to have).