Each search term position in the result of the search engines (this is called SERP) can be monitored over time. Currently only Google and Yahoo are monitored (both control around 90% of the search engine users).
Results are presented in graphical charts that provide easy view of historical data.
Each of the desired search terms will be searched for in the 200 first positions in Google and Yahoo.

You can choose to see the results on either Yahoo or Google.

Note:
This feature will only be useful for registered clients (signup is free), since historical data will only be collected for registered clients.