As far as what it is and why it is important:

Zero crossing is the moment when an AC signal crosses the zero voltage
point, which occurs 120 times per second in USA AC powerlines (60hz sine
wave), and 100 times per second elsewhere.

There are two reasons to know the zero crossing,
First, to minimize noise in a switching system, turn it on at the zero
crossing.  If you turn a circuit on at a non-zero crossing you end up
with a current spike in the line.

Secondly, Triacs and SCRs (cheap high current switching devices) don't
turn off until the current going through them is zero.  So you can turn
a triac on 1/4 way through the AC cycle and it will stay on until the
next zero crossing (with a few caveats linked to reactive loads).

You can see a tutorial on AC light dimming at
http://ubasics.com/adam/electronics/doc/phasecon.shtml
which includes pictures and descriptives on zero crossing and phase
control.  No zero crossing circuits yet, but there is one on Steve
Walz's archive at
ftp://ftp.armory.com/pub/user/rstevew/CCTS/xSSRelay.cct

-Adam

Edson Brusque wrote:
>     While we're off topic, I want to make good use of the opportunity and
> ask how all this "zero crossing" thing works, for what is this for and
> examples of detection circuits.
>
>     Thanks for all,
>
>     Brusque