Optimizely Classic was the original Optimizely platform, dedicated to A/B testing with some enhanced features that aided Conversion Rate Optimization.
Optimizely X takes the game to a whole new level. With modules like Optimizely X Web Experimentation, Optimizely X Web Personalization, Optimizely X Web Recommendation and Optimizely X Full Stack, you get the tools you need to run a highly advanced conversion rate optimization program, and conduct experiments on any code base you want (not just your website).