Following, Muting & Blocking

When you see someone, you can click on their user picture to view their profile, and click on the userpic in that to see full profile. You can follow them, mute and block them.

Following is self-explanatory, it adds them to your Home Timeline, lists you as a follower and gives you access to follower-only posts if they have any.

Muting collapses posts and notifications made by them, giving you an option to see the post if you're curious. Clients other than the web interface may completely remove their posts.

Blocking a user removes them from your timeline and notifications and prevents them from following you (automatically unfollows them from you).

Please note that some users can be "locked", meaning instead of following them you send a follow request they need to approve for you to become their follower.