In Mountain Lion, Apple split off its reminder options from the provided calendar program and put them into a separate program similar to the Reminders app in iOS. This program lets you keep track of your projects, routines, or what have you, with to-do lists for each. As you make progress, you just mark each reminder as completed, and it drops off of its respective list.
But if, say, you inadvertently mark the wrong reminder as completed, don't worry, you can still get it back.
The Reminders program does have a Completed section that you can click to see all completed reminders, but this is a global list and includes the reminders for every project you have a list for, instead of just those for a given project or routine.
The completed items for a specific list of reminders are relatively hidden at the top of the current reminder list; if you scroll up, you will see a Completed section that shows the completed items just for that list. As with the global Completed list, this will show the list of items along with when you completed them. This essentially gives you a timeline of when you completed each of your various tasks.
In addition to being useful for organization, this is where you can quickly find and resurrect an item inadvertently marked as completed. Just scroll up and uncheck your item rather than having to create a new reminder to replace it.