Just wanted to chime in that @mendes1 and @thomas a reed has given great answers to the question and add my own view on it.
OSX sometimes caches too zealously which results in the usage of too much ram leaving not enough ram for actual apps that the user is actively using. For example, downloading a 21gig file from the app store is taking me about 10 days to do so given the speed of the net where i'm at. I leave it running whenever I don't need the bandwidth for other stuff. To my surprise, leaving it downloading for over 30 mins would result in all my free ram gone and Free Memory App's auto-purge when Free Mem drops to 20 megs would occur which would then freeze the machine for a few seconds. Why would downloading a huge app from the app store require caching in ram at all? Save batter power? Seriously the basic idea is sound on how Apple implements caching and has resulted in a lot of performance improvements but sometimes there are just plain silly situations.
Lastly without knowledge of purge or Free Memory App, I've seen the macs of friend basically freeze and required force quitting apps they were actively using to get control of the system back. I've since taught them to buy the app or use purge periodically