Whenever we feel sad, annoyed or frustrated over things that stress us out, we forgot to control the words that comes out in our mouth. We simply utter words without thinking first. Sometimes these words we speak could hurt someone else, just like daggers and knives without us knowing what we have done.
There are times in our lives where we tease our friends and call out stupid name or teasing them by highlighting how fat or thin they are. These words may seem natural to us and we simply neglect their reaction to it. But in reality, no matter how they hide and act like they are not affected by these, no matter how friendly the way the approach is, the feeling is still there. They are also humans who get hurt when insulted.
The simple tease you say would hurt someone with you having no clue about it, especially when it comes from a person they think the least to do it. You never knew that the person you call thin might be suffering from some serious gastrointestinal diseases that prevents him from gaining weight or simply his family could not afford the luxury to buy more food. You never knew that the person you call fat is trying hard to lose weight to the point he/she would skip meals. We never knew what is under the mask they wear until we are on their stead.
These simple jokes and teases could possibly grow larger and will eventually be considered as verbal bullying

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What is Verbal bullying?

Verbal bullying or verbal abuse is a type of bullying that is done through speaking or statements with the intention to humiliate and insult people by saying negative words. Unlike, jokes and simple tease, the abuser will not apologize implying that the action was intentional and was meant.

Why people do it?

If words could hurt people, then why do it at the first place? There are different reasons why people continuously do such thing. These are some of the most common reasons:

They do it because they didn't know that they are teasing them more than enough.
They are enjoying other people's reaction or the attention that was resulted from the insult.
Envy. They feel jealous about other people's achievement.

What you feel after the insult?

The feeling that the person who is doing the insult will be totally different from the people who got acted upon.
The bully might feel satisfied or simply not care about what happened. For him, it is just another joke that do nothing against other.
But for the person who got acted upon, a lot of emotion will swirl up within him.
These are some of the possible outcome of the action:

Depression
Low self-esteem
Grudge
Upset
Possibly bully others

What to do when you see someone get bullied or when you are getting bullied?

When we see someone get bullied, the first thing we think about is to just forget about it. This is inevitable especially when the bully is bigger than us. But this is not the right thing to do. Think about the person who got bullied and suffer from the action.
First and probably the best thing to do for the victim and witness is to report it to the teachers or to the authorities. They are the most likely the ones who can help the victim immediately. Next, if you are the one who is bullied, tell your parents about what is happening. Don't get afraid to tell them because they are the ones you could hold on to.
Next, don't ever stop going to school. This is one way to tell that you are being affected by the bully. Be strong and face them.
Last, be positive and don't let the negativity get you. Ignore what the bully say. Those are simply words that they see about your outside looks. This does not define you or who you are.