There are two types of sound cards that are generally used in Gaming PC builds i.e. Internal Sound Cards and External Sound cards. The first one sits on the motherboard right inside your pc case while the latter can be connected using a USB interface. There are advantages and disadvantages of sound cards  (both variants) which completely depend on the gaming requirements.
Most preferred among Gamers are the internal sound cards as they are connected directly to the motherboard using PCI/PCIe port. It does not require any extra interface in between. Internal Sound cards come in both 5.1 and 7.1 channel output. As compared to external sound cards, these are less expensive and efficient. Manufacturers like Asus, Creative etc are the most popular internal sound card brands.
These sound cards are very simple plug and play devices and installation is very easy, unlike internal sound cards. They use a USB 2.0 interface to be connected to the PC. Both 5.1 and 7.1 channel options are available in external sound cards. The issue with an external sound card is that they have latency associated with them to an extent.