There are currently six different types of soundcards available and it can be very confusing trying to determine what type is suitable for you. The following is a quick run down of the pros and cons of each type.
PCI Interface - 1056 megabytes per second
PCI (Peripheral Component Interconnect) is an interconnection system between a computer and attached devices. All standard modern desktop computers have PCI slots built into the motherboard ranging from 1 - 10 available slots although the majority of PCs have about 4 slots. When compared to other interface types PCI cards can transmit more information quicker but this should not be confused with latency values when using music software. The possability to transfer data quickly means manufacturers can offer more features on the card such as a large number of channels of input and output. The limited size of a PCI card means manufacturers can't fit too many features on the cards themselves. To get round this they sometimes include a breakout box which is basically a box that connects to the PCI card to allow additional I/O sockets and features such as microphone pre-amps. Since PCI cards need to be installed inside a computer you can not install a PCI card in a laptop.
PCMCIA - 1056 megabytes per second
PCMCIA stands for Personal Computer Memory Card International Association, an international standards body and trade association that was founded in 1989 to establish a standard for connecting peripherals to portable computers. PCMCIA created the PC Card also known as 'PCMCIA cards' which are credit-card sized, removable modules for laptops. They are basically the laptop version of the PCI card as described above. Again due to the very limited size of these cards they are often packaged with a breakout box to provide additional features.
USB 1.1 Interface - 12 megabytes per second
Universal Serial Bus is a general purpose peripheral interconnect mechanism which has become a standard on modern computers. Version 1.1 of this protocol is only able to transmit 12 megabytes of information per second which is much less than either PCI or PCMCIA cards. Up to date computers will have USB 2.0 ports which means they can accept either USB 1.1 devices or USB 2.0 devices, it is the device that dictates the version used, not the ports. The advantage of an external USB device is that there is no size restriction since it is already a type of breakout box. USB1.1 devices are limited by the amount information they can pass to and from the computer. As such they are restricted to two channels of simultaneous input and two channels of simultaneous output. This configuration is often enough for the novice home user and therefore manufacturers tend to use the USB1.1 protocol so that they can keep the price of the units low. However, if you require more than 2 channels of simultaneous input or output then you should not look at USB1.1 devices.
USB 2.0 Interface - 480 megabytes per second
Version 2.0 of the Universal Serial Bus is capable of transmitting much more information than version 1.1 and this means more channels of audio input and output can be utilized. However sound-card manufacturers are favoring the FireWire communication protocol since this was designed from day one as a general-purpose serial bus for connecting high-speed peripherals to a computer. There are currently only two devices which use the USB 2.0 communication protocol.
FireWire 400 Interface - 400 megabytes per second
FireWire 400, also known as IEEE 1394,, is a general-purpose serial bus for connecting high-speed peripherals to a computer. It originated as Apple Computer's serial bus. FireWire is a standard on Apple Mac computers but it is not yet included as standard on the majority of desktop PC computers. You can add FireWire ports to your desktop PC or laptop PC by purchasing a FireWire PCI or PCMCIA card for about £20. FireWire is capable of transmitting a large amount of information per second therefore it is used by sound-card manufacturers so that they can offer devices with multiple channels of input and output. There has also been a trend for manufacturers to use FireWire for more basic devices. If you are looking for an external device with more than 2 channels of simultaneous input or output then you should look at FireWire 400 devices. There are two types of FireWire ports, 4 pin ports and 6 pin ports. If your computer only has a 4 pin FireWire port then you will not be able to power the audio interface directly from the computer, you will need to plug it into the mains.
FireWire 800 Interface - 800 megabytes per second
FireWire 800, also known as IEEE 1394b, is the next generation of FireWire technology. Since FireWire 400 already offers enough bandwidth for complex audio interfaces, manufacturers don't tend to use the FireWire 800 protocol. In fact there is currently only one device available which takes advantage of the extra speed. FireWire 800 use 9 pin ports which is different to FireWire 400's 4 pin or 6 pin ports.