Games are rated based on how well they run on my computer. Note that the rating is specifically based on performance and not how good a game it is.

 

The rating scale is: 

Great: Everything runs as smooth as you would expect on a true gamer PC. Irrellevant things like videos might lag.

Good: Mostly a good performance though there might be slowdowns from time to time. Most irritants should be avoidable by lowering some settings like graphical settings or number of players.

Okay: Though you lower settings and number of players you won't be able to get good performance. But the game is still playable. Might contain instructions for what settings to use in order to get the best performance.

Bad: Sluggish performance that severely hampers the playability

Unplayable: Don't even try. Either you won't even be able to start the game or you'll be presented to a slideshow.