1. A method of classifying web visitor sessions based on clickstreams, the method comprising: organizing a plurality of web pages of a website into a plurality of web page categories; dividing a clickstream of each visitor to visit the plurality of web page categories of the website into a plurality of visitor sessions; determining, by at least one processor of a machine, using a visitation metric based on the plurality of web page categories, a mathematical distance between each of the plurality of visitor sessions; and classifying each of the visitor sessions into one of a target group and a non-target group based on the mathematical distance between each of the plurality of visitor sessions and on an identification of at least one of the visitor sessions with an event corresponding to the target group.