Definition: CAML - Collaborative Application Markup Language: is an XML based markup language used in SharePoint to define Fields, Views, Templates, Forms, etc. used in SharePoint Sites and Lists.  

Mostly developers use CAML to query and fetch records from list and libraries. 

CAML is case-sensitive so make sure while Defining or Querying using CAML.

There are some free tools available to build CAML query. Developers can utilize them to construct query and use them in their projects.