If you've been using a joystick, bots, GPS spoofers or any of the other cheats, hacks and mods out there to reduce the amount of actual walking you have to do in Pokémon Go - rather defeating the point of the game - then today is not going to be your favourite day. Niantic has said that on August 18, the bans would start going out for the latest wave of cheaters and supposedly it's going to be quite comprehensive.
The particular people Niantic is going after with this ban wave, are those who have been using third party applications to play. Niantic has been shutting down those tools left and right to try and keep a lid on it, but banning many of the high-level players who take advantage of them will probably have more of an impact in the long run.
However, some are concerned about how serious Niantic is taking this. While it would obviously ban accounts for using botting tools or GPS spoofers, the concern is that even using IV checking sites, where you confirm how powerful your Pokémon could become, would also be considered a use of third party tools and could result in a ban.
Fortunately Niantic is leaving a door open for those who feel they were wrongly banned. In the end of the email going out to banned users, it says "if you believe your account has been terminated in error, you may fill out this form." It then links users to a particular complaint page, though Niantic does not have a strong history of fulfilling support requests.