Well it varies. Generally when you get a "polygon" limit it's in Quad faces. I normally see triangle limits expressed explicitly as tri's and comes up more in the game industry as most game engines expect triangulated meshes.
The motion picture and comcercial marketing industries tend to use quads, but someone like Jay would be more qualified to comment on that as he has worked for a couple studios..
The problem with saying polygon is that technically a polygon can have any number of sides although the accepted norm is to model in quad's.
However, I have seen studio models used in both TV and major motion pictures that are rife with n-gons and tri's.
If you are just starting out then strive to model in quads. You will learn over time where you can get away with the occasional triangle and even the dreaded ngon.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675