Many species have gone extinct, an estimated five billion, over the course of history. Humans are to blame to for the loss of many organisms, some of which have been plentiful at the time we turned our attention toward them. Some think that dozens of species are going extinct every single day. Maybe we could undo it for some of them, however.
First, we have to have a good genetic sample to bring an animal back to life. That puts dinosaurs in the highly unlikely category of animals that we'll see roaming the planet again; we haven't obtained a robust, complete DNA sample from a dinosaur. But, we do have woolly mammoth genome samples.
Giving life to extinct animals would require cloning. With an excellent DNA sample and some edits, we can make the necessary modifications so an elephant would be able to carry a wooly mammoth-elephant hybrid to term, and give birth to a successful organism. That animal would then have to live somewhere, something that would absolutely have to be carefully considered before any we start resurrecting extinct species. Check out the video for more discussion.