What is the difference between rejecting and deleting a post?

There is a difference between rejecting and deleting a post.

Rejecting a post

If you reject a post, the post is saved in the Search archive and can be found by filtering "rejected" posts.

Deleting a post

If you delete a post it is completely erased and cannot be found in Search.

User Access: Only superusers with Inbox access can reject and delete posts.