Contao uses content elements and frontend modules to generate content. Toolkit uses the term components to describe them. Toolkit allows to develop lightweight components. They does not require to extend from the Contao class stack. Toolkit also provides a way using dependency injection for components.
If you’re developing for Contao 4.5 you might look at the fragments support. If you have to stay on Contao LTS 4.4 you maybe want to read further.