After the failure of Justice League, Warner Bros is now not focussing on the shared universe much (unofficially called DC Extended Universe or DCEU). So a lot of seemingly independent movies are in the offing.
Todd Phillips' Joker, which stars Joaquin Phoenix in the title role, is not related to the larger DC film universe, which reportedly still has Jared Leto in the role of the Clown Prince of Crime. It releases on October 4, 2019.
Todd Phillips' Joker is an origin movie, and will trace the journey of Joaquin Phoenix's the Joker from a comedian to the Clown Prince of Crime. It will be more of a dark character study than the usual comic-book caper.