What is user provisioning and de-provisioning in Active Directory.

Provisioning often means more than just creating a user in AD. It's setting everything that needs to be done to get the user going, i.e. giving appropriate access rights, adding to groups, moving to appropriate OU, creating a mailbox, creating a home folder, enabling for all associated systems in the environment, etc. So basically it's everything that happens between the moment when a user comes in the the company and the moment when everything he requires from the IT systems is ready.

Deprovisioning is same thing but in reverse, i.e. procedures between the moment a user decides to leave and the moment that all IT stuff is properly revoked, disabled, archived, etc.

Depends on who is using the term. One meaning is to allow someone to manage active directory (creating and deleting users, etc) through a website. I created a website about 8 years ago that did that and the term we used was Provisioning.