Ooh, long S! (not exactly the same as F, just very similar!: ſ only has half a cross-bar on the left*, f has it all the way across). For some reason people decided that S should look different depending on its position in the word—the basic principle is that ſ is used at the start and in the middle of words and s at the end, but there are lots of complications and exceptions, and the use wasn't always consistent. I've found a blog post that goes into a lot more detail!

ETA: *and it evidently depends on the font, because the one Dreamwidth uses for posted comments clearly doesn't have a cross-bar at all... :D