vmstat Field Descriptions

   Procs

       r: The number of processes waiting for run time.
       b: The number of processes in uninterruptible sleep.

   Memory

       swpd: the amount of virtual memory used.
       free: the amount of idle memory.
       buff: the amount of memory used as buffers.
       cache: the amount of memory used as cache.
       inact: the amount of inactive memory. (-a option)
       active: the amount of active memory. (-a option)

   Swap

       si: Amount of memory swapped in from disk (/s).
       so: Amount of memory swapped to disk (/s).

   IO

       bi: Blocks received from a block device (blocks/s).
       bo: Blocks sent to a block device (blocks/s).

   System

       in: The number of interrupts per second, including the clock.
       cs: The number of context switches per second.

   CPU

       These are percentages of total CPU time.
       us: Time spent running non-kernel code. (user time, including nice time)
       sy: Time spent running kernel code. (system time)
       id: Time spent idle.
       wa: Time spent waiting for IO.

Analysis

Sustained high swap rates (si and so) are usually bad.  The system will start spending all of its time swapping, and make no progress on any actual work.  You will also see the number of runnable (r and b) processes increase.  If the situation gets bad enough and free memory gets too low, the Out-of-memory (oom) logic will start killing random processes.  At this point, either reducing the number of processes that normally run or adding additional RAM are about the only options.