A thesaurus presents keywords in their mutual relationship. You will also find variants of the keywords (see: Keywords).
When you look up a term, you will also see what the broader term (BT=Broader Term), narrower term (NT=Narrower Term), related term (RT=Related Term) or preferred term (UF=Used For) is. This is very useful if you do not yet know exactly which word to search for.
This is an example from the NHL Stenden catalogue 300.6 (social science research).
Once you know the right code for your topic, you can click on that code in a library catalogue to find other publications on that topic.
There are many different classification schemes. The best-known general scheme is the SISO. This is the scheme used by the NHL Stenden library and most public libraries.