PBS has a full list of Nature programs focused on endangered species. You can watch many of them right from your computer or portable device. In addition, there are other resources available to extend your knowledge about the animals featured in this series.
How many species exist on the earth today? You will want to read all about what a challenge it is to answer that question. From the National Wildlife Federation.
The two federal agencies that oversee endangered species in the United States are the U.S. Fish and Wildlife Service, for land-based species (including those that live in land-based waterways such as lakes, ponds, and rivers), and the National Marine Fisheries Service, for those that live in the oceans.