If anyone knows about the Nasuverse well, Rouge Servants are Masterless Servants. They are usually Servants who began operating independently after their Master died, or ones that accidentally appeared here without any particular reason.
That certain justice guy who is a Ruler was once a Rouge Servant, but because he had no Master, he was quite weak.
The information might be wrong, but eh, this is the Fate. Even the people inside the setting aren't entirely sure about the massive, sprawling amount of Fate lore, so what can you expect from the person who created this bootleg Holy Grail?
(If Jalter here possesses a Servant body, then there's a high chance she is also a Rouge Servant.)
Alright, back to the main topic.
Alter Servants.
Explaining them is quite simple. They are essentially alternative versions of the original Heroic Spirit — histories that shouldn't have existed, but were created due to some kind of external influence that branched off into an alternate timeline.
And yeah, most of them aren't exactly good people. They are either:
1. Extremely negative,
2. Have extreme goals,
3. Or are just airheaded like that certain Japanese girl.
Except for some cases like Jalter, because she only shares Jeanne's appearance and memories but is almost completely unrelated. She even only appeared after Jeanne had died to emphasize that the two really have nothing to do with each other.
The most typical examples of Alter Servants are: Black Saber, Black Saber but riding a horse with big boobs, Black Saber but riding a motorcycle and wearing a bikini, Black Saber but Santa, Edgy black justice boi, Absolute India god, the nonchalant katana illness girl, berserker not really furry girl, unfriendly face France queen, Darth Vader, Idol Darth Vader...
As for Cú Alter, I don't know, just ignore him.
The easiest way to recognize them is that, overall, they are weaker than their original canon versions in some aspects (except for a few).
Black Saber is corrupted so she can't use Excalibur's true power. In exchange, she is the one who made Gilgamesh simp for her and acts like a very strong husband toward the justice boi. Oh, and she really likes burgers too.
Edgy black justice boi is much weaker compared to his red version because he can almost never use his Reality Marble. In exchange, he uses GUNS! (America yeehaw!) And he's allowed to use the N-Word... Oh, and he is literally a killing machine. He does it even better than Red Archer in some aspects.
Actually, Red Archer really shouldn't have an Alter version because he's currently still working as a janitor, plus he doesn't have any notable reputation in history (except for the train running through the universe, those round cats with world domination goals, 7749 fanfics where he becomes mommy — and I swear I never get tired of reading his fanfics).
So having an Alter version is really ridiculous. He's even immune to the Holy Grail's mud — well, actually in FGO he does get corrupted, BUT he still doesn't change from his original self so... ehhh whatever.
So who is that edgy guy?
How should I put it? It's true that the current Red Archer won't be affected and will never become an Alter because he's a janitor.
But the problem is his younger version. Because that's when he hadn't actually died and become a Counter Guardian yet.
Yeah, I'll just leave it at that so you can find out yourself. There's even a manga about him anyway (I wish there were fanfics about him. I hope Nimthewriter writes something about him, even just a little. I'm waiting for the new chapter of his Fate crossover fanfic).
Ahem, I got a bit too excited there. But well, that's mama we're talking about, how can you blame me?
Alright, thank you for reading this unnecessary information. If you're a Nasu fan familiar with his works (especially the VN), you don't need to read this because it's all the most basic info.
And if you're a true fan of the Fate series, don't play FGO. It's not worth your time. That garbage game will only make you want to commit suicide.
See ya.