USB uses tree topology.

USB hubs
  Requires a controller card (primary hub).

  Hubs can be used to extend distance.
    Limit of 5 hubs daisy-chained. Max of 30M.

  PC capable of multiple independent hubs.
    Two PCs cannot be hooked together with only USB.

    Special cabling allows 2 hubs or PCs to interact.

  USB-on-the-Go designed to allow peripheral devices to connect to each
    other without a root hub. (2003?)

  Additionally, there are any number of USB to alternative interfaces.  
   USB to :
    parallel (printer)      serial (RS232)    SCSI
    Ethernet                TV/Video          Keyboard/mouse.