Zoo is an extremely aggressive aggro deck that utilizes
low- 
cost 
,
self- 
boosting 
creatures 
along with
burn 
and
utility spells 
. It's main strategy is to simply beat its opponents into submission before they can actually get anything going.
Your average zoo list used to look something like this:
Spoiler:
Show
It's a fairly straight-forward deck so there aren't all that many interactions to explain. The main ones though are these:
Knight of the Reliquary +
Fetch 
lands 
:
Knight of the Reliquary can easily become very large, very fast. By continually dumping fetch lands into your graveyard or sacrificing lands to the
Knight you can have a 5/5 or 6/6 creature for
fairly early in the game.
Turn one
Fetch land into a
dual 
land 
followed by either
Kird Ape or
Loam Lion (or in the past,
Wild Nacatl ) meant that you had a 2/3 creature on turn one. That puts your opponent on an extremely short clock and you could conceivably kill them by turn four or five.
Other than that there aren't that many interactions in a regular Zoo deck. People used to use
Punishing Fire 
and
Grove of the Burnwillows 
in some variants but with
Punishing Fire now banned, there isn't really any point in explaining that.
There are other variants of Zoo such as
Tribal Zoo 
,
Dark Zoo 
(Jund?), etc. They're all pretty much just variations of a basic Zoo frame. I don't play Zoo, so I'm probably not the right person for explaining this. You might poke around the Modern forums for Zoo decks or maybe check out a primer somewhere for a more in depth analysis. Anyway, hope that helps.
