But here's what little I know.
The original Wonder Girl concept was accidental. DC was telling stories with kid sidekicks and baby versions of their popular characters, so Wonder Woman, Wonder Girl and Wonder Tot all interacted...even though writers were never clear on whether they were all younger versions of Diana or independent characters, separate from her. The Teen Titans concept was also around at the time, and took off a bit, so Wonder Girl there was clearly becoming her own person. They called her Donna Troy, and said (I think) that she was a baby rescued by WW in a house fire, that was then brought to Paradise and trained by the Amazons.
Perez worked on the Teen Titans book before Crisis, and it was really successful...so after Crisis supposedly rebooted everything, DC still wanted to keep the Titans going without restarting them much and confusing things. I didn't read this stuff, so I'm going off what I read online here.
Bottom line was, Donna Troy, aka Wonder Girl became Troia, and had that short haircut and black outfit with red and blue and gold accents. In spite of the fact that Wonder Girl should have logically come *AFTER* Wonder Woman, in the Post-Crisis timeline Diana was entering Man's World for the first time in the then-contemporary 80s...and she was new. No one knew who she was. And yet...Donna Troy/Troia existed somehow. So...yeah. That was weird. There are a few Perez issues where they meet face-to-face and the discrepancy is kind of explained away, but it confused the heck out of me so I just ignored it.
Bottom line is - Donna is one of the most confusing characters ever because she's been killed like a dozen times and brought back with different origins and then killed. In the Perez era, I believe she's supposed to be a literal mirror image of Diana, created by an old Amazon witch (why there even IS an Amazon witch I have no clue, as they're all supposed to be ageless, right?) named Magala, who frees young Diana's reflection from a mirror one day so the young Princess will have a playmate on the island. Yeeeaaaah.
Anyway, in the 90s, during the Byrne run, Cassandra Sandsmark is introduced. She's pretty much a copy of Perez's character Vanessa Kapatelis but without the narrative baggage. Anyway, she worships Diana and wants to be like her, so she steals the gauntlet of Atlas and the sandals of Hermes, which give her strength and enable her to fly. She wears a black wig to cover her blonde hair, so she'll look more like WW and goes out to fights crime. Diana at first says no, but then softens and agrees to train her. She becomes the "modern" Wonder Girl, eventually losing the black wig and rocking the blonde hair and red outfit look. She gets into a relationship with Conner Kent, aka Superboy.
In the meantime, Donna continues to get killed and resurrected. She drops the Troia name and just goes by Donna Troy.
Cue the New52 reboot, which brought back Cassie as Wonder Girl but changed her powers or personality or something. I don't know. Look that one up yourself LOL.
At present, post-Rebirth, they seem to have de-aged Cassie a bit (she looks about 14) and gone with a simple jeans and hoodie style costume. Cassie is still pretty popular as Wonder Girl but fans are pissed she has been sidelined and rewritten in recent times.
It looks like Donna Troy is still around, and fought in the VERY recent event Trial of the Amazons, during which Queen Hippolyta died and the remaining Amazons battle for the crown, including Donna, Artemis, Diana and long-forgotten Amazon Nubia, who appeared in the 70s, was forgotten and now is being brought back.
OH - and there's Yara Flor, the NEW Wonder Girl, introduced like two years ago. She's Brazilian and there's some link to the Amazon rainforest there but...don't ask me what it is. Her solo series was cut short, bizarrely and the announced television spin-off was axed. She got a lot of press and big push right out the gate before anyone even knew who she was as a character. Now, she's in a supporting role it seems.