To buy the right hard drive it is important that you know the interface type of your computer. The interface type determines the compatibility to a system. Common interface types today are IDE (also known as PATA), SATA, or SCSI. SCSI interface type may come in different pin configurations.
Speed and Capacity determine how much data can be stored and how fast the data can be accessed. The higher the capacity, the more data can be stored. Similarly, the higher the speed, the faster the computer can access or write data to/from the hard drive.