What is the difference between archiving and leaving a chat?

 
When a chat is  archived, it will be removed from your regular All tab and moved to the Archived tab in your inbox. You will still receive new messages in that chat but it will stay clear of your inbox. This is a personal setting and will only affect your own chats and inbox.

When you  leave a chat, you will no longer receive new messages in that chat. To rejoin a chat, contact one of the chat admins. They can add you to the chat again.

  More about Chat actions.