What is Telegram and why was its CEO arrested in Paris?

Pavel Durov, the founder and CEO of the messaging app Telegram, was arrested in Paris over the weekend over allegations that his platform is being used for illicit activity such as drug trafficking and the distribution of child sexual abuse images. In a statement posted on its platform, Telegram said Durov “has nothing to hide and travels frequently in Europe.” The news put the spotlight on Telegram, a popular app that allows for one-on-one and group chats as well as large “channels” that let people broadcast messages to subscribers.