Political views and ideas which are far from the mainstream are called extremist. Such views often violate common moral standards. As an example, a group that wants to replace a democratic form of government with a totalitarian one is almost always considered extremist. Political or religious fundamentalism as well as fanaticism can also be seen as extremist.
What is seen as extremist varies by country. People who support that Muslim women should wear a burqa may be seen as extremist in France, but such an opinion may not be extremist in Afghanistan or Saudi Arabia. extremism is really dangerous