What are Novichok agents and what do they do?

Traces of Novichok were found in a restaurant and a bar near where Skripal and his daughter were found, but the area was thoroughly decontaminated. Even so, the couple that fell ill Saturday were in Salisbury a day earlier, near roads that had been sealed off during the Skripal investigation, according to the Independent. Counterterrorism police are investigating a possible connection between the two incidents.