Sort of Recommender Systems

1. Collaborative filtering methods 
1. Memory-based methods
1. User-based collaborative filtering
2. Item-based collaborative filtering
2. Model-based methods
2. Content-based recommender methods 
3. Knowledge-based recommender systems
4. Hybrid systems

All of them are based on:

• Agent interactions – dynamic or time oriented
• Item attributes – static or features oriented
• Ratings matrices