What's the difference between a good and bad things

Good things are things that help things, improve things, etc.

Bad things are things that are against the law, changes things for the worst, etc.

Good vs Bad is a broad subject and is hard to determine at times. Something that's good to one person might be bad to another.
But at the end of the line it depends on consequences, and who or what was affected and how.
Usually the option that leaves the most people and\or things with more of a positive outcome is "Good" and the option that has a more negative effect on people and\or  things is "Bad". Also actions that are selfless are considered good.

For instance, if you decide to help someone with a chore, (say cleaning a window) you might not have fun doing it but that other person doesn't have to spend as much time on their chore and also feels better the whole day because someone was nice to them. It would also be considered selfless so it makes it even better of a thing to do.

But if you make the window that the person is cleaning dirtier for fun it might make you feel good because it was fun,
but now that person has to spend more time cleaning the window and they would also feel bad because someone was mean to them and is considered "Bad"

It's a whole different story if you are talking about actual "things" as in objects or natural occurrences.
But the bottom line is.
Good = more positive outcome than negative.
Bad = more negative outcome than positive.