The sound card is not a very essential part to your computer. It is basically there for entertainment purposes only. It processes all of the sound information that comes through your computer, then sends it out to your speakers. If you have a microphone that will also plug into the back of your sound card. Most sound cards also have a game port on the back of them where you can plug a joystick or controller into them. Using the game port is outdated though, nowadays gaming devices are usually USB.
The sound card shown here is a Sound Blaster Live! MP3+ 5.1, what the 5.1 means is that it can send individual signals out to 5 speakers and 1 subwoofer. For an example if you wanted surround sound with this card, you would have a front left speaker, front right, rear left, rear right, center, and a subwoofer. A 4.1 system would be without the center speaker, which is pretty useless anyway. The cables that you see to the left are for CD audio, and Digital CD audio. the cable on the bottom is for Digital, which is used in DVD drives. The top cable is the CD audio cable for either a CD or a DVD drive. If you want to listen to CDs in your pc you have to have one of these cables going to your sound card. That is about all you really have to know about a sound card. You do not need to spend a bunch of money on one. I would have to say that a model that is few years older would work just as well.
I gave Creative Labs Sound Blaster Live! MP3+ 5.1 a 96% for greatness. It is a really good sound card and it came with a bunch of good software. There is really nothing bad about it, except for the fact that it gives me trouble when I hook the modem to the sound card so I can call somebody on the phone, but I think that is probably a software issue. I purchased this excellent card online for $70, it was a great deal with the software bundle that I got with it, but if you don't need the software you should be fine with the Sound Blaster Live! Value sound card, for about $40.