The link @TheNumb provided explains what's going on here. The Linux kernel will use any available memory for disk caching, unless it's required by a running program. If needed by a program, the memory will be freed up. From the site Linux Ate My RAM:
Why does top and free say all my ram is used if it isn't?
This is just a misunderstanding of terms. Both you and Linux agree that memory taken by applications is "used", while memory that isn't used for anything is "free".
You would call that "free", but Linux calls it "used".
Memory that is You'd call it Linux calls it taken by applications Used Used available for applications, and used for something Free Used not used for anything Free Free This "something" is what top and free calls "buffers" and "cached". Since your and Linux's terminology differs, you think you are low on ram when you're not.
