When attendance is enabled for an event, not only can you see who is planning on attending an event, but you can also mark whether or not they actually did attend.
 

Roles needed: Site Admin, or Attendance admin.

Via the Event's Page

You can view your events via My Events, the calendar, or the Group Agenda. When on the page for an event, click the "attendance" tab along the top edge.
You will be brought to a page that shows the current attendance plans in a table view along with the actual attendance. This is called the summary view.

To view plans in detail, you can click the button just above labeled "plan".
 

To take attendance, click on the "actual" button. This will bring you to a page with the names of attendees listed in a table, with their plan in one column, and then the option to set their actual attendance in the next.

Sometimes it's easier to make everyone present, and then change the attendance of a few who are missing. You can do so by expanding the "select all" button that's just above the table. The rows will highlight yellow when selected. Then click the "Actions" button and choose the state to which you want to change these records. The page will then update. Please note, you can use the "select all" button, and then click a row to un-select it if you want to select most users, but leave a few out.

You can then make any individual changes, including adding notes if you know why someone is absent.
When you're done, click the "Done taking attendance" button. The numbers on the summary page and other attendance overviews will update, and the editable section of attendance will become static.

Of course, if you need to make any changes, you can always click the button "reopen attendance".
 

Via the Attendance Summary

You can also take attendance via the attendance summary page by clicking on the "attendance" link on the far right of an event listing. Then, the rest of the steps are the same as above.

Note: The edit link is to edit the event itself, not the attendance.