Because humans are ruining animal's homes or habitats, they are killing those species. If those species get killed until the point that there are no more that is called extinction. Extinction is when an animal does not exist anymore either because it's habitat got destroyed, humans over hunted it, or by natural cause like earthquakes, tornados, or floods. But most of animal extinctions have come from humans. Sometimes they don't even notice that they are, but that doesn't mean it's not important.
Endangered means that there is only a small amount left of that species. There are tons and tons of endangered animals beacause of the pollution and litter and people ruining their homes. We are trying to save these animals by putting them in zoos and making them reproduce so in time hopefully we will let them back into the wild. Sure a lot is being done about this, but even so a lot is not being done.