Apex invoking means executing the Apex code. Apex class execution takes place through one of the ways listed below.

• The anonymous block and triggers
• An event that fires a trigger
• An asynchronous Apex
• A scheduler or batch job that runs Apex classes at specified intervals
• Web Services class
• Email Service class in Apex
• You can expose your Apex methods via SOAP and REST Web services with Apex Web Services
• Visualforce Controllers
• Apex Email Service is used to process inbound emails
• The JavaScript Method of Invoking Apex
• The Ajax toolkit to invoke Web service methods implemented in Apex

Below is the example of a lightning component invoking apex class.

Aura Component.

Apex Controller