Memory Usage with cat /proc/meminfo

The entries in the /proc/meminfo can help explain what's going on with your memory usage, if you know how to read it.
Example of 'cat /proc/meminfo':

total:  used:    free:  shared: buffers:  cached:
Mem:  1050001408 1012899840 37101568        0 113672192 420950016
Swap: 2097434624 217985024 1879449600
MemTotal:      1025392 kB
MemFree:         36232 kB
MemShared:           0 kB
Buffers:        111008 kB
Cached:         279304 kB
SwapCached:     131780 kB
Active:         677908 kB
ActiveAnon:     487272 kB
ActiveCache:    190636 kB
Inact_dirty:    129164 kB
Inact_laundry:   23948 kB
Inact_clean:     15332 kB
Inact_target:   169268 kB
HighTotal:      131008 kB
HighFree:         2336 kB
LowTotal:       894384 kB
LowFree:         33896 kB
SwapTotal:     2048276 kB
SwapFree:      1835400 kB
Committed_AS:  1079884 kB

The information comes in the form of both high-level and low-level statistics. At the top you see a quick summary of the most common values people would like to look at.

total:    used:    free:  shared: buffers:  cached:
Mem:  1050001408 1012899840 37101568        0 113672192 420950016
Swap: 2097434624 217985024 1879449600

Below are the individual values. First we will discuss the high-level statistics.

MemTotal:      1025392 kB
MemFree:         36232 kB
MemShared:           0 kB
Buffers:        111008 kB
Cached:         279304 kB
SwapCached:     131780 kB

High-Level Statistics

• MemTotal: Total usable ram (i.e. physical ram minus a few reserved bits and the kernel binary code)
  
• MemFree: Is sum of LowFree+HighFree (overall stat)
  
• MemShared: 0 is here for compat reasons but always zero.
  
• Buffers: Memory in buffer cache. mostly useless as metric nowadays
  
• Cached: Memory in the pagecache (diskcache) minus SwapCache
  
• SwapCache: 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)

Detailed Level Statistics

VM Statistics

VM splits the cache pages into "active" and "inactive" memory. The idea is that if you need memory and some cache needs to be sacrificed for that, you take it from inactive since that's expected to be not used. The vm checks what is used on a regular basis and moves stuff around.
When you use memory, the CPU sets a bit in the pagetable and the VM checks that bit occasionally, and based on that, it can move pages back to active. And within active there's an order of "longest ago not used" (roughly, it's a little more complex in reality).

Active:         677908 kB
ActiveAnon:     487272 kB
ActiveCache:    190636 kB
Inact_dirty:    129164 kB
Inact_laundry:   23948 kB
Inact_clean:     15332 kB
Inact_target:   169268 kB

• Active: Memory that has been used more recently and usually not reclaimed unless absolutely necessary.
  
• Inact_dirty: Dirty means "might need writing to disk or swap." Takes more work to free. Examples might be files that have not been written to yet. They aren't written to memory too soon in order to keep the I/O down. For instance, if you're writing logs, it might be better to wait until you have a complete log ready before sending it to disk.
  
• Inact_clean: Assumed to be easily freeable. The kernel will try to keep some clean stuff around always to have a bit of breathing room.
  
• Inact_target: Just a goal metric the kernel uses for making sure there are enough inactive pages around. When exceeded, the kernel will not do work to move pages from active to inactive. A page can also get inactive in a few other ways, e.g. if you do a long sequential I/O, the kernel assumes you're not going to use that memory and makes it inactive preventively. So you can get more inactive pages than the target because the kernel marks some cache as "more likely to be never used" and lets it cheat in the "last used" order.

Memory Statistics

HighTotal:      131008 kB
HighFree:         2336 kB
LowTotal:       894384 kB
LowFree:         33896 kB
SwapTotal:     2048276 kB
SwapFree:      1835400 kB
Committed_AS:  1079884 kB

• HighTotal: is the total amount of memory in the high region. Highmem is all memory above (approx) 860MB of physical RAM. Kernel uses indirect tricks to access the high memory region. Data cache can go in this memory region.
  
• LowTotal: The total amount of non-highmem memory.
  
• LowFree: The amount of free memory of the low memory region. This is the memory the kernel can address directly. All kernel datastructures need to go into low memory.
  
• SwapTotal: Total amount of physical swap memory.
  
• SwapFree: Total amount of swap memory free.
  
• Committed_AS: An estimate of how much RAM you would need to make a 99.99% guarantee that there never is OOM (out of memory) for this workload. Normally the kernel will overcommit memory. The Committed_AS is a guesstimate of how much RAM/swap you would need worst-case.