At the top right of f.com?
I suppose “new” topics are ones that have been started recently, despite the amount of replies they’ve garnered.
“Active” topics could have been started a few years ago, but are still attracting interest and formenting debate/discussion.
I think the only difference is that “Active Topics” is easier to click on than “New.”
It’s a bigger target for my mouse/cursor team, and it’s not so darn far away. 
New threads are ones that have had a post added since you last looked.
Active topics have had more than a certain number of posts added over a 24 hour period (4 or 5, I guess… don’t know for sure).
I liked the old ‘24’ view, so you could see the first 3 lines of a posts so you didn’t need to open 'em. What happened to that? I suppose you get fewer hits, if you have a feature like that, though - not good for advertising.