I’m sure this is a very silly question, but after going round in circles trying to answer it for myself, I’ve decided to ask anyway. 
Firstly, I am not on Facebook, have never used it and know nothing about it.
However, I have a client who has been asking periodically, for about the last year, for a “Facebook button” on her web site. I do understand that there are various types of button - like/share/follow, etc. Every time I ask her which type she wants, things go quiet for a month of two. I’m pretty sure she’s as clueless about this as I am, and just wants “a button” because “everybody has one”.
I’ve finally reached the point where I think all she wants is a link from her website to her Facebook page - nothing more. Now that I can do - but where do I find the icon to use? Is there somewhere on Facebook where I can download it? Because if there is, I’ve failed to find it. 
A Google image search brought up a bewildering array of icons, each slightly different, and I’ve no idea which, if any, is the “official” one. (And yes, I know I can’t just use an image because it’s come up in a search.)
If you’re ever round this way, I’ll bake you a cake.