Bicycle chains are what drives the bike forward. It is the connection between your pedalling and the rear wheel, moving you along and making sure your drivetrain does its job. Choosing your chain depends on how many gears you have, as you need to match your bicycle chain to how many gears you have in the back.
The width of your chain will change with an 8 speed chain, a 9 speed chain, or a 10 speed chain and 11 speed chain. There are even 12 speed chains available now. Lighter chains have hollow pins holding the links together, and higher performing chains feature specific shapes to make shifting that much smoother and faster. Many chains have quick links that make breaking the chains and putting them together again easier.