Otherwise known as USB, this has become a very simple and well developed way of connecting computer devices. From the name, you may deduce that it operates with a Serial Data Stream protocol.
USB has a fairly detailed standard, and beginners should probably start with all-in-one chipsets and consider Jan Axelson's "USB Complete".
A connection following the USB standard dictates that one end will be a "host" and the other a "device". Only a host may have devices connected to it. For example, a PC is a host, and a USB hard drive would be a device. It is for this reason that the Lego NXT cannot be connected to a USB 2 hard drive. If a connection with a protocol that treats devices as peers is required, USB is not suitable.
One big advantage to USB is that it was designed to be Hot Pluggable though do not presume every device works this way. Some operating systems will offer to restart when you plug in a new device anyway.
USB Standards stipulate the basics of the electronic connection, timing, power, device identification and the signalling. It also has standards for Disk-like devices, and human-interface devices like keyboards and mice. By producing hardware that conforms to these standard specifications, a manufacturer can save time by not needing to produce drivers. This is useful to a robot builder, because it means a lot of assumptions can be made about a device if it conforms.
Phidgets are a range of actuators that can be connected directly to a computer. They could be used with a laptop or MiniMac to produce a rather large and expensive robot. While they are handy, and simple, they are very costly, and require an on board computer.
With no exceptions, a <a a="" brain")="" class="wiki" for="" href="/wiki/microcontroller.html" robot"="" title="A programmable digital controller (or ">microcontroller</a> and a set of servo's and motors is a more flexible, smaller and cheaper (by orders of magnitude) option, although they do take a little more effort. That even includes kits as extensive as the Lego NXT
When evaluating <a a="" brain")="" class="wiki" for="" href="/wiki/microcontroller.html" robot"="" title="A programmable digital controller (or ">microcontrollers</a> that boast USB, be clear on whether that is USB host, or USB device. Only a host may have devices attached to it, and most controllers are merely devices. As mentioned above, the NXT allows a PC to upload to it, or control it via USB, but would not be usable with Phidgets.
Time to build your own robot! Using a Raspberry Pi with parts and some time, you can use my book to learn how to make and program a robot with automatic behaviours.