free utility shows the amount of free / used memory.
The biggest misinterpretations that I’ve seen on both Windows and Linux systems are with the swap and cached memory.
Cached memory is exactly what it sounds: memory dedicated for cache. Cache from this memory can be dropped at any time when needed, for example if some process needs a big chunk of memory, the kernel can simply give it some of the cached memory.
swap is a part of disk that is used when physical RAM memory is full. Unlike Cached memory, Swap will never use caching.
So, in plain English: available memory is both cached memory and used, while swap is only used.
free shows the following information:
|total physical memory: RAM + SWAP
|used memory excluding buffered/cached memory
|sum of memory used by tmpfs file system
|memory dedicated for caching that can be released if needed
|actually available memory
Available memory includes both free memory and buffered/cached memory.
Total memory is obviously total: RAM and SWAP.