什么是Cohort？
A group of users who share a starting point in time, such as everyone who signed up in the same week, tracked together over time.
A cohort is a group of users defined by a shared starting point, most often time-based: everyone who signed up in the week of March 3, everyone whose first visit was in January. Instead of one blended number across your whole user base, cohorts let you follow each group forward and compare their behavior at the same age.
The classic output is a retention grid: rows are signup weeks, columns are weeks since signup, cells are the percentage still active. Read down a column to see whether newer cohorts retain better than older ones, which is the cleanest evidence that a product change worked, since each cohort experienced a different version of the product. Blended metrics hide this completely: a growing site can show flat overall retention while every new cohort is actually improving, because new users always retain worse than long-tenured ones.
Cohorts group users by when they started; segments group them by who they are or what they did. The two combine well, for example retention cohorts filtered to one acquisition channel.