Over 99% of all living creatures that have ever lived are now extinct and no longer walk the earth. That comes to a total of about 5 billion species. Scientists estimate there are almost 9 million species still alive today. Many of those animals are being discovered yearly in some of those places that we haven't been able to search in the past. Locations like the Amazon, the Congo, and the deep sea. There are many reasons an animal may go extinct. In the past there have been major extinction events like weather changes and natural disaters. Most often times, the repercussions of one of these events causes a species' environment to change or even disappear. Sometimes the animal cannot adapt to their new surroundings, or they can't find a new environment. This leads to a major species death and usually the extinction of the kind.
Nowadays, extinctions happen over time. Fast and sudden species deaths don't occur very often. In the last 100 years there have been about 500 confirmed extinctions. Most of the recent extinctions have happened because of over hunting, poaching, pollution, and habitat loss. There has been very little regulation on logging and hunting. Extinctions have a big impact on the environment. If a species dies in an ecosystem where it is a staple in the food chain, other animals will suffer and possibly die out as well. If an apex predator(an animal that is at the top of the food chain) goes extinct, another species whoose population used to be kept in check may overpopulate and die out as well.
At the top of the page are links to some species that have died out. Most of them all went extinct for
different reasons and in different times. The last page contains species that are not yet gone but are
trending in a bad direction.
Here is a video from Animal Planet going more into depth on what extinction is and it's impact on nature.