Mainstream political parties are established and widely accepted parties within a country's political landscape. They often have a long history, broad support base, and generally subscribe to moderate and centrist policies.
Radical populist parties appeal to the "common people" and often position themselves in opposition to the "elite" or "establishment". They can be found on both the right and left of the political spectrum, depending on the context of the nation they are in. Their policies can be radical or unconventional compared to mainstream parties.