1. 定义
协同过滤（Collaborative Filtering）有狭义和广义两种意义：
广义协同过滤：对来源不同的数据，根据他们的共同点做过滤处理。
Collaborative filtering (CF) is a technique used by some recommender systems.协议的协同过滤：它假设如果用户A和B对一个话题Y有相同的兴趣，那么A和B对另一个话题X都感兴趣的概率比随机抽取两个人且都对话题X感兴趣的概率高。所以它能根据收集到的用户兴趣预测一个用户是否对某个话题感兴趣。
In the newer, narrower sense, collaborative filtering is a method of making automatic predictions (filtering) about the interests of a user by collecting preferences or taste information from many users (collaborating). The underlying assumption of the collaborative filtering approach is that if a person A has the same opinion as a person B on an issue, A is more likely to have B's opinion on a different issue x than to have the opinion on x of a person chosen randomly.
下面这个图非常形象的说明了协议的CF的工作原理：