Chains are perhaps the most important of all cycling components, with the machine unable to work without one.
Your bike could have a top of the line Ultegra groupset, but without a good quality chain to run on it that counts for nothing. A good quality chain is important for improving the longevity of the components.
The size of the chain depends on the kind of cassette you are running, as different speeds will require different chain lengths. MOst chains are able to be easily adjusted with a chain break tool, which you can find in this section also.
To prolong the life of the chain and save yourself some money on cycling components, regularly clean and regrease the chain especially if you ride it off-road or in wet conditions, as the grime that accumulates can ruin your drivetrain as well as the chain itself.