From ARS Technica:

Clickstream data includes every web site visited by each user and in which order they were clicked.
The data is not sold with accompanying user name or information, but merely as a numerical user value. However, it is still theoretically possible to tie this information to a specific ISP account. Cancel told Ars that his company licenses the data from ISPs for millions of dollars. He did not give a specific figure about what this broke down to in terms of dollars per ISP user, although someone in the audience estimated that it was in the range of 40¢ per user per month