Last visit is the last time someone was active on a site.

Last activity, the last thing they were doing...

You capture them by using sessions and storing the info in a database each time the user moves between pages.

Say they move from page x to page y.... When they arrive at page y, your script stores the time they loaded the page (last visit) and stores their last activity (give the page an activity name) by storing what they were doing in the database.