Omniscient is one of several POV choices you can make when writing a story. An omniscient narrator has complete access to all characters' actions, dialogue and inner thoughts, so readers aren't viewing the action through the eyes of one character as in first or third person narration. Your narrator is like a god, seeing everyone and everything.
Point of view (or POV) tells us whose head we are in when we read a story and how the story is being told. If your viewpoint character narrates the story using 'I', 'me' and 'my' personal pronouns, you've got a first-person POV. If an external narrator is telling the story, referring to your protagonist (and other characters) with 'he', 'she' or 'they', it's third-person POV.
Protagonist is another word for ‘main character’ in a story.
YA, short for 'young adult', is a category of fiction aimed at teenagers, typically aged between 13 and 18. It’s far less restricted than middle grade in terms of acceptable content and stories often tackle bigger themes and emotional conflict.