How does Lethality Work?

Ok, so I've been gone for a while, and I come back to see that quite a lot has changed. But, not least of which, is this new "lethality" thing. I'm a little confused about it, honestly.

Firstly, it's applied to some items, instead of others (it's on and a few others but get to keep their armor pen? I thought riot was trying to get rid of armor pen?) and secondly, I don't really understand the UI's notes on these items.

So, I see people building Duskblade and Youmuu's all the time, but each has a unique passive of +18 lethality... I mean, it's not named like other unique passives, but it is identical... do they stack? I mean, I recently tried out a lethality build on just to see what all the hubbub is about, and I didn't really notice a huge difference to my damage output after buying my first item. I built Duskblade, got a pretty big damage boost, but then a I bought Youmuu's a little later and didn't notice that much more damage on the tanks I was trying to kill. I mean, it could be because I was kind of behind because I was playing SUPER careful in the early game, but I was wondering.... is stacking these lethality items actually doing anything? I figured I'd get a straight answer before I try the build again.

Also, the UI doesn't help very much, because I can't tell if it's saying how much armor each individual item negates, or how much armor your build as a whole negates. I couldn't tell.

Also, if anyone could explain this, a major point of confusion I have about lethality is.... what does it actually do? It says "negates armor" - how is this different from armor penetration? I'd love to see the numbers on this if I could. Because as far as I can tell, it's just supposed to be a sort of "scaling armor penetration" of some kind, but I could be wrong.

Also - apologies if this has been explained before - I did a quick forum search and couldn't find anything on it other than threads where people are talking about how broken lethality is right now? I'm not sure about that... maybe I'm just bad at these "lethality champions" or something.