全然違う means "totally different". For example :「日本語は英語と全然違います」- Japanese is totally different from English. 「この式は全然違います」- That formula is totally different. 等の文脈に使うと思います。私が間違ってるなら直してください。

Nelly's answer is perfect. 全然違う means "totally different" or "completely different".