What does 'Blender's System supports only quads' mean? It is stated in a lot of website that blender's system supports only quads. But blender also renders polygon. So, what does this mean? And how is it different from programs that supports polygons(example wings 3D)?
First let's solve some term confusion here. Every face of a mesh is a polygon. A tri is a polygon with three edges. A quad is a polygon with four edges. And a N-gon is a polygon with n edges. n stands as a variable here for any possible value. In 3D a N-gon describes a face with more than four edges. So when somebody talks about a N-gon then you know: he talks about a face / polygon with more than four edges.

Now for your question: The statement that Blender supports just tris or quads is not longer true. Since Blender can deal with N-Gons too since a pretty while. The code part that made this possible is called Bmesh. Which is a special code part under the hood of Blender to deal with the mesh geometry. It gots introduced with Blender 2.63. More to read here: https://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.63/BMesh