You can tell who Skaar is from the title of his 2008 solo comic series, Skaar, Son of Hulk. More properly, he's the son of the Hulk and Caiera, the Hulk's wife from the Planet Hulk storyline. Surviving his mother's death, his attempts to save his planet from Galactus backfired and saw him exiled to Earth, where he fought his father before falling in with Bruce Banner, who has decided to train him so that he'll be able to kill the Hulk when they next meet.
As the Hulk's son, he has the same enhanced strength and healing power as his dad, but is into tattoos and bad rock music.
The result of the Hulk's DNA being stolen by a time-traveling feminist who came from a future where no man was worthy to father her child and sabotage had left technology unable to propagate the human race - I promise, I am not making this up - Lyra has returned to the present day to assure the future of her kind, Terminator-style, by breeding with Norman Osborn. Understandably, she realized that wasn't really something she wanted to do, but decided to stick around in this timezone to fight crime and find some other way to keep the future (and her past) safe.
Unlike her father, Lyra actually gets weaker as she gets angrier, as the result of genetic manipulation by her mother's race, the Femizons. Yes, the Femizons. I refer you to the part about me not making this up.
After Skaar had left to go to Earth, Hiro-Kala was revealed to be the other Son of Hulk. Raised unaware of his parentage and as a slave, Hiro-Kala inherited his mother's powers, and has revealed himself to be an able and more-than-a-little-ruthless little bastard; he managed to avenge Galactus' destruction of his home world by tricking him into eating another planet that he'd "poisoned". Sure, destroying an innocent civilization and planet for revenge may seem extreme, but it got the job done.
As opposed to all of the other Hulks, Hiro-Kala isn't on Earth right now - which is probably good, or else he might poison here as well - and hasn't met any of the rest of his family yet.