The actual "core-default" Elgg friendship uses the same model as Twitter: the relation is not reciprocal and it's not required to confirm a friend request. It is really "following" rather than "friendship". The interesting thing is that there are plugins to change it to the reciprocal/confirmation model of Facebook/LinkedIn.

When you friend someone you can then see their activity in your friends section of the river.  You can only see activity that they marked as available for public, logged in users, friends (if they added you as a friend), or a custom friend collection.

Friedship is one of the options you can choose to restrict access to the areas with a customizable access level, such as blogs, groups, pages, activity and so on.