As a person who's lived with them all my life and seen the evolution of
the sand-mount system, here's what I know about them:

Waste leaves the house to a large tank. The waste (solids and liquids)
enter at one end of the tank where the solids settle to the bottom,
floating "stuff" stays at the top, and in the middle is the liquid.
Solids break down over time in the bottom of the tank and are
reabsorbed eventually. The liquid leaves at the opposite end of the
tank, having passed around one or more baffles to keep the floaters and
solids from leaving the tank. The second tank accepts the flow from
the first tank and contains a pump. Any solids that make it past the
first tank will be given a chance to settle out here as well. When the
level gets high enough, the pump switches on and pumps the liquid out
to the sand mound. There is a distribution box that the liquid is
pumped into and splits into multiple pipes that are buried in the sand
mound called leach lines. The pipes are perforated to allow the liquid
to seep into the sand, which will filter the wastewater. Some of the
liquid evaporates, some of it seeps through the sand back into the
environment.

You should have the tanks pumped out regularly. Depending on tank size
and how much water your household uses: every 2-5 years. The solids
have a way of building up in the bottoms of the tanks over time, since
they don't really break down fast enough to keep from filling up. The
pump should be good for 10 years or so I'm told. There should be an
alarm in the system to let you know if the liquid level gets higher
than the pump float (indicating that the pump isn't working). I'm not
sure what your vents are in the sand bed, but I suspect it may be one
or more manholes for access to distribution box(es).

Here's a decent site I found once to describe different systems:
http://www.eco-nomic.com/indexsdd.htm#Mound%20Systems
Here's one with more info, including a handy link to an estimate of how
often to pump your tank: http://www.inspect-ny.com/septbook.htm