What actually is bullying?

I've been so confused on whether what I've experienced or not is actually considered being bullied or it's just people being mean for the sake of being mean.

For alittle bit of context, I've been best friends with this girl for about 4 years. She always got verbally bullied like almost everyday. Like actually almost everyday. She's moved since last year because they ended up throwing gum in her hair and that was the last straw. Since then, they've made some comments towards me but it isn't what they did to her.

In my point of view it's more them being annoying more than anything, but I talked to my friend about it and she said that it's bullying.
For example to paint a scene of what theyare doing: two dudes who are apart of the same friendgroup of those people, constantly making small comments to me and another friend in class. Asking if I'm dating him because "girls and guys can't be friends." Which is stupid bc their friendgroup is mostly girls and they aren't dating any of them.
There were some other small comments and them constantly asking stupid questions to me. I can't say it doesn't effect me but it's honestly just more being tired at their stupidity. I don't even react to it so I don't know why they keep trying.