General purpose port.

  Wide range of controls and broad protocol.

  Bi-directional data transfer or 'lanes' with paths in both directions.

  Common ports
    PCI-e - hardware point to point. Software multi-port.
      internal - but external extension available.
      PCI-e uses lanes, 2 pair of lines make up a lane.
        one pair for sending, one for receiving.
        Lanes are expanded in powers of 2,   1, 2, 4, 8, 16, 32
        16x max on most PCs.
      
      * bottom slot is standard PCI interface.

    SATA - storage device interface, Hard drives, CD, DVD, Blu-Ray, SSD
      Limited protocol. internal.
      

      eSATA - external interface.
     


    USB - hardware point to point. Software multi-port. External.
      Requires master.

    IEEE 1394 (Fire-wire or i.LINK) - alternative to USB. External.
      Initially more advanced, built on SCSI protocols. 
      Uses arbitration - any device may be master.
      

    Thunderbolt - combination of Display Port and PCI-e and DC power. 
      External.
    
      Up to 4 lanes of data transfer.
      2 duplexed lanes (10 Gib/s per lane) in Thunderbolt mode.
        (20 Gib/s max in each direction).
      Originally designed for optical cabling but copper works fine.

 
 * Some of these can use hub or daisy chain implementation allowing a
   single port from controller to multiple devices.
   Addressing usually implemented via software packets.