I will answer this question into 2 categories. One is based on the general information that most people know, and the other is based on my own experience.
Bullying is the use of force, threat, or coercion to abuse, intimidate or aggressively dominate others. Others may verbally bully others, some may physically bully people, and some may also exclude other people (ignoring and snickering when they're around).
Why do people bully others? People bully each other because the person bullying might be jealous of them. Also, they have trouble at home and let it all out on them to make them feel better. Or, they just want to impress someone or show off. But most of the time they end up looking like an idiot and are called immature as some people may say.
Now based on my experience, I think that bullying can be more than just words or insults. It can be even more than a physical fight. Bullying can start from a simple glare at a person which can escalate to almost everyone looking at you like that. Bullying can also be about you reputation. Others may spread slander about you and it may get to the point where your teachers, or even your own parents may believe such rumors.
Now as for why people bully others, I think it could be because the bullies were once bullied before. They could have been bullied in elementary which is why in the future, they might want others to feel that way as well. They could also feel insecure on the inside which is why they bully to make them feel like they have power and are powerful over others.
Well there you have it! I hope that this helped you in any way. Feel free to contact me if you have any other question. Have a great day!