/proc/meminfo provides information about distribution and utilization of memory. This varies by architecture and compile options.

cat /proc/meminfo
 
MemTotal:     16344972 kB
MemFree:      13634064 kB
Buffers:          3656 kB
Cached:        1195708 kB
SwapCached:          0 kB
Active:         891636 kB
Inactive:      1077224 kB
HighTotal:    15597528 kB
HighFree:     13629632 kB
LowTotal:       747444 kB
LowFree:          4432 kB
SwapTotal:           0 kB
SwapFree:            0 kB
Dirty:             968 kB
Writeback:           0 kB
AnonPages:      861800 kB
Mapped:         280372 kB
Slab:           284364 kB
SReclaimable:   159856 kB
SUnreclaim:     124508 kB
PageTables:      24448 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
WritebackTmp:        0 kB
CommitLimit:   7669796 kB
Committed_AS:   100056 kB
VmallocTotal:   112216 kB
VmallocUsed:       428 kB
VmallocChunk:   111088 kB

where:

• MemTotal: Total usable ram (i.e. physical ram minus a few reserved bits and the kernel binary code).
• MemFree: The sum of LowFree+HighFree.
• Buffers: Relatively temporary storage for raw disk blocks shouldn't get tremendously large (20MB or so).
• Cached: In-memory cache for files read from the disk (the pagecache). Doesn't include SwapCached.
• SwapCached: Memory that once was swapped out, is swapped back in but still also is in the swapfile (if memory is needed it doesn't need to be swapped out AGAIN because it is already in the swapfile. This saves I/O).
• Active: Memory that has been used more recently and usually not reclaimed unless absolutely necessary.
• Inactive: Memory which has been less recently used. It is more eligible to be reclaimed for other purposes.
• HighTotal:
• HighFree: Highmem is all memory above ~860MB of physical memory. Highmem areas are for use by userspace programs, or for the pagecache. The kernel must use tricks to access this memory, making it slower to access than lowmem.
• LowTotal:
• LowFree: Lowmem is memory which can be used for everything that highmem can be used for, but it is also available for the kernel's use for its own data structures. Among many other things, it is where everything from the Slab is allocated. Bad things happen when you're out of lowmem.
• SwapTotal: Total amount of swap space available.
• SwapFree: Memory which has been evicted from RAM, and is temporarily on the disk.
• Dirty: Memory which is waiting to get written back to the disk.
• Writeback: Memory which is actively being written back to the disk.
• AnonPages: Non-file backed pages mapped into userspace page tables.
• Mapped: Files which have been mmaped, such as libraries.
• Slab: In-kernel data structures cache.
• SReclaimable: Part of Slab, that might be reclaimed, such as caches.
• SUnreclaim: Part of Slab, that cannot be reclaimed on memory pressure.
• PageTables: Amount of memory dedicated to the lowest level of page tables.
• NFS_Unstable: NFS pages sent to the server, but not yet committed to stable storage.
• Bounce: Memory used for block device “bounce buffers”.
• WritebackTmp: Memory used by FUSE for temporary writeback buffers.
• CommitLimit: Based on the overcommit ratio ('vm.overcommit_ratio'), this is the total amount of memory currently available to be allocated on the system. This limit is only adhered to if strict overcommit accounting is enabled (mode 2 in 'vm.overcommit_memory'). The CommitLimit is calculated with the following formula:
  
  • CommitLimit = ('vm.overcommit_ratio' * Physical RAM) + Swap
    
  • For example, on a system with 1G of physical RAM and 7G of swap with a `vm.overcommit_ratio` of 30 it would yield a CommitLimit of 7.3G.
    
  • For more details, see the memory overcommit documentation in vm/overcommit-accounting.
    
• Committed_AS: The amount of memory presently allocated on the system. The committed memory is a sum of all of the memory which has been allocated by processes, even if it has not been “used” by them as of yet. A process which malloc()'s 1G of memory, but only touches 300M of it will only show up as using 300M of memory even if it has the address space allocated for the entire 1G. This 1G is memory which has been “committed” to by the VM and can be used at any time by the allocating application. With strict overcommit enabled on the system (mode 2 in 'vm.overcommit_memory'), allocations which would exceed the CommitLimit (detailed above) will not be permitted. This is useful if one needs to guarantee that processes will not fail due to lack of memory once that memory has been successfully allocated.
• VmallocTotal: Total size of vmalloc memory area.
• VmallocUsed: Amount of vmalloc area which is used.
• VmallocChunk: Largest contiguous block of vmalloc area which is free.