Coronaviruses are a large family of viruses known to cause diseases in mammals and birds. Most do not evolve to infect and spread between humans, but some have managed to cross over. Diseases that do this are known as zoonotic.
In humans, these viruses cause (typically) mild to moderate respiratory infections, like the common cold. Two such coronaviruses in recent memory, MERS and SARS, have been known to cause severe illness and can be fatal.
The most recent coronavirus to make the news is the 2019 Novel Coronavirus, or COVID-19. It was first detected as an outbreak of respiratory illnesss in Wuhan, China. Early patients were believed to be connected to a "wet market" in the region that sold wild animals for food. Subsequent patients had no exposure to the market, which indicated person-to-person spread.
On March 11, 2020, the World Health Organization officially declared it a pandemic.