Illegal and banned are two different things. In the US there are no "bans" like the way China does things. The Government can't "ban" web sites, they can just make it illegal to display/publish certain material - but if someone decides to break the law there is nothing in place to automatically prevent them from posting it, or others from viewing it. The few things I know are outright illegal: Child porn, bomb making guides, corporate/trade secrets, intellectual property (without permission), ANY threats against the President or other elected officials, very specific (actionable) threats against anyone else. Private web hosting providers can and often do enact limits and outright bans on certain content they don't want to provide, but that's their right as a private business. The Government can't tell them what they can and can't host.