There are a number of types of speakers that you can buy for your computer. Typically, they are rated by number, with the first number representing the amount of satellites associated and the second number (after the decimal) representing the amount of subwoofers. Your audio needs and how much you're willing to spend are the best factors in determining which type of speaker is right for you.
A 2-speaker system is one of the most common types of speaker setups. It offers moderate quality sound and doesn't take up any space.
The 2.1 speaker setup involves 2 satellite speakers and 1 subwoofer. This setup offers a much fuller spectrum of sound while still saving space. This system is also most appropriate for music.
5.1 speaker setups consist of 5 satellite speakers and 1 subwoofer. These systems are the most common surround sound setups in use today. Offering multi-channel output, surround sound systems are excellent for movies and video games, as they give the viewer a more "engulfing" audio experience.
The 7.1 speaker system is one of the most complex systems, consisting of 7 satellite speakers and 1 subwoofer. These allow for some of the most precise surround sound environments provided that the source media can output that level of accuracy--otherwise the output will actually output 5.1 surround sound. Keep in mind that 7.1 systems will cost you: they start around $300.
More recently, wireless speakers have allowed users to avoid the mess of wires associated with surround sound speaker setups. While these are excellent for many users, they can often conflict with a home Wi-Fi network.