Covid-19: Rapid antigen vs PCR tests

DIY Covid-19 antigen rapid test kits being unpacked before being put on display for sale in a Singapore pharmacy. Such kits are useful for their quick results, although they are not as sensitive as PCR tests. — Reuters

On June 10 (2021), Singapore’s Health Ministry announced that DIY (do it yourself) Covid-19 antigen testing kits would be available for public to buy off the shelf for self-testing purposes from June 16 (2021).

But what is a Covid-19 antigen test, and how is it different from a Covid-19 PCR (polymerase chain reaction) test?