Watch and Friend
Watch
Any account you "Watch" will then send you Notices about submission uploads, journals, streams, etc by that account.
You can watch an account by going to the Userpage for that member, and clicking the "Watch User" link at the top of the page.
Friend
Any member you "Friend" will then have access to your private content, and you will have access to theirs.
For a friend request to be accepted between two members, they must both send/accept a friend request.
You can friend an account by going to the Userpage for that member, and clicking the "Friend User" link at the top of the page. This will send a "friend request" that they will then have to accept before your accounts become linked as Friends.
To accept a friend request, go to the Userpage of the member making the request and click "Accept Friend".