A large family of viruses. There are many different kinds and some cause disease. They are known to cause illness ranging from the common cold to more severe and often fatal diseases such as MERS, SARS, and most recently COVID-19.
Coronaviruses are larger than most viruses and are named for their spiky crown-like appearance (Corona in Latin means crown). First described in 1968, coronaviruses can mutate very quickly and have, on several occasions, jumped the species barrier, spreading from animals like birds, bats and pigs to humans (see Zoonoses/Zoonotic diseases).
The coronavirus family is not new to humans. There are hundreds of them, and we have lived with them for centuries. They usually cause mild upper respiratory tract infections, like the common cold. However, the SARS-CoV-2 virus has claimed over a million deaths globally in a matter of months.