The two common USB standards differ greatly in terms of transfer rates. USB 1.1, which has been around for many years, offers transfer rates of up to 12 mbit/s. USB 2.0, which only came out 18 months ago, is growing in popularity thanks to its transfer rates of up to 480 mbit/s.
Since the USB standard was designed to be an interface for all kinds of peripherals, it was planned from the very beginning that devices would be run on USB hubs. All you have to do is hook up the hub to an USB port on your computer and voila! You now have several more available ports. The advantage of this set-up is that you can run more devices than you have connectors. There is a downside, though: you may encounter problems when using high-performance USB devices because the bandwidth has to be divided up among the different devices.
To understand why performance problems are not uncommon, you need to know something about the USB protocol. USB devices can access any of the four sub-protocols: bulk, control, interrupt and isochronous. They help allocate the "attention span" (read: bandwidth) a device needs to operate.
No matter what else happens, 10% of the bandwidth is reserved for the control protocol, which directs all the transfers. Audio or video devices such as webcams or speaker systems work isochronously ("in real time") because they always need a minimum amount of bandwidth. Most external storage devices use the bulk protocol, while USB keyboards or mice avail themselves of the interrupt protocol.
No matter what, 10% of the USB bandwidth goes to the USB controller.
You've probably already guessed the problem: whenever you connect two USB devices that both use the isochronous protocol and take up a certain amount of bandwidth, you'll have very little left over for any other devices. USB is a first-come-first-served standard, so if you connect a third or a fourth device to the hub, you may not have enough bandwidth left over to operate it.