Interface
Serial ATA, USB, FireWire
Internal Internal enclosure refers to all hard drives that are installed inside desktop or laptop computers. External External hard drives are those that are set up outside of a computer, usually connected via a wire. Plug-In Plug-In Modules are those hard drives that are essentially removable or swappable.
The interface is the type of connection (such as FireWire or USB) used by your hard drive. Home systems will mostly use an IDE interface. Professional systems can use higher quality interface, as SCSI.
Interface (Detailed)
USB, FireWire 400 (1394a), USB 2.0, Serial ATA, FireWire 800 (1394b)