In Linux, what is the difference between "buffers" and "cache" reported by the free command?

The "cached" total will also include some other memory allocations, such as any tmpfs filesytems. To see this in effect try:

mkdir t
mount -t tmpfs none t
dd if=/dev/zero of=t/zero.file bs=10240 count=10240
sync; echo 3 > /proc/sys/vm/drop_caches; free -m
umount t
sync; echo 3 > /proc/sys/vm/drop_caches; free -m

and you will see the "cache" value drop by the 100Mb that you copied to the ram-based filesystem (assuming there was enough free RAM, you might find some of it ended up in swap if the machine is already over-committed in terms of memory use). The "sync; echo 3 > /proc/sys/vm/drop_caches" before each call to free should write anything pending in all write buffers (the sync) and clear all cached/buffered disk blocks from memory so free will only be reading other allocations in the "cached" value.

The RAM used by virtual machines (such as those running under VMWare) may also be counted in free's "cached" value, as will RAM used by currently open memory-mapped files (this will vary depending on the hypervisor/version you are using and possibly between kernel versions too).

So it isn't as simple as "buffers counts pending file/network writes and cached counts recently read/written blocks held in RAM to save future physical reads", though for most purposes this simpler description will do.

Tricky Question. When you calculate free space you actually need to add up buffer and cache both. This is what I Could find

A buffer is something that has yet to be "written" to disk. A cache is something that has been "read" from the disk and stored for later use.

http://visualbasic.ittoolbox.com/documents/difference-between-buffer-and-cache-12135

I was looking for more clear description about buffer and i found in "Professional Linux® Kernel Architecture 2008"

Chapter 16: Page and Buffer Cache

Interaction

Setting up a link between pages and buffers serves little purpose if there are no benefits for other parts of the kernel. As already noted, some transfer operations to and from block devices may need to be performed in units whose size depends on the block size of the underlying devices, whereas many parts of the kernel prefer to carry out I/O operations with page granularity as this makes things much easier — especially in terms of memory management. In this scenario, buffers act as intermediaries between the two worlds.