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# free
             total       used       free     shared    buffers     cached
Mem:       2059064    2027988      31076          0     195060    1223508
-/+ buffers/cache:     609420    1449644
Swap:      4194296     213160    3981136


The second number on the line that starts with "-/+ buffers/cache:" is the amount of free physical memory you have on the system in kilobytes. This number is also ram that isn't being used for disk caching by the kernel. The 3rd number on the line saying 'Mem:' is the amount of physical memory that hasn't been allocated by the kernel for any reason.

If you are confused about what is meant by allocated versus used, think of it this way. You buy a house. You buy a piece of property and build a house on it. That house could use almost all the property, but it doesn't. This is like physical memory allocated by the kernel. Its best not to use all the property and leave some left over just in case or for a tree or something. Now the house itself has a set number of square feet to it. You start buying stuff and putting it in the house, but you don't completely fill the house. This is like ram that is used for programs. Memory used for disk cache doesn't really have a good metaphor in the house analogy and neither does swap, unless you live in America and have one of those offsite self storage things.