interpreting memory usage

[R Kimber]

Last night, after a reboot and after opening a few standard apps, free
gave

           total       used       free     shared    buffers cached
Mem:2055816     482572    1573244   0 36560     179136
 -/+ buffers/cache:   266876    1788940
Swap:  2104464   0    2104464

i.e. it seemed to be saying that 76.5% was free.
This morning, with the same apps remaining open, it gave

            total       used       free     shared    buffers cached
Mem:2055816    1704252     351564          0 364044     248908
 -/+buffers/ cache:    1091300     964516
Swap:      2104464          0    2104464

i.e. 17.1% free.

How should this be interpreted?  It seems a big drop in "free"
memory, whatever that means, given that no new apps were started and I
did very little on the machine during the intervening period. The
buffers and cached went up from about 10% to 30%

Are there system/housekeeping processes running in the background that
aren't started at boot and that can take up such a large chunk of memory
(presumably around half a GB)?  Or am I misinterpreting the
figures?

Subjectively I think that 5.04 uses more memory than its predecessor,
the desktop seems quite expensive in memory terms, and to get slower
after a while.

I ask this not because I'm concerned about the problem but because I'd
like to understand a little better (I normally use zero swap and I know
that linux manages memory well, though the slowdown is an issue that I
shall keep an eye on).  

- Richard.
-- 
Richard Kimber
http://www.psr.keele.ac.uk/