Understanding Edges and Faces in Google SketchUp 8
In SketchUp, everything is made up of one of two kinds of things: edges and faces. They’re the basic building blocks of every model you’ll ever make.
Collectively, the edges and faces in your model are geometry. When someone refers to geometry, she’s talking about edges and faces. Other modeling programs have other kinds of geometry, but SketchUp is pretty simple. That’s a good thing; there’s less to keep track of.
A basic cube drawn in SketchUp is composed of 12 edges and 6 faces. The other model is a lot more complex, but the geometry’s the same; it’s all just edges and faces.
SketchUp’s edges: The basics
Edges are lines. You can use lots of tools to draw them, erase them, move them, hide them, and even stretch them. Here are some things you ought to know about SketchUp edges:

Edges are always straight. Not only is everything in your SketchUp model made up of edges, but all those edges are also perfectly straight. Even arcs and circles are made of small straightline segments.

Edges don’t have a thickness. This one’s a little tricky to get your head around. You never have to worry about how thick the edges in your model are because that’s just not how SketchUp works. Depending on how you choose to display your model, your edges may look like they have different thicknesses, but your edges themselves don’t have a builtin thickness.

Just because you can’t see the edges doesn’t mean they’re not there. Edges can be hidden so that you can’t see them; doing so is a popular way to make certain forms. Even organic shapes and curvy forms are made up of straight edges.
Working with faces in SketchUp
Faces are surfaces. If you think of SketchUp models as being made of toothpicks and paper (which they kind of are), faces are basically the paper. Here’s what you need to know about them:

You can’t have faces without edges. To have a face, you need to have at least three coplanar (on the same plane) edges that form a loop. In other words, a face is defined by the edges that surround it, and those edges all have to be on the same, flat plane. Because you need at least three straight lines to make a closed shape, faces must have at least three sides. There’s no limit to the number of sides a SketchUp face can have, though.

Faces are always flat. In SketchUp, even surfaces that look curved are made up of multiple flat faces. You can see that what looks like organically shaped surfaces are really just lots of smaller faces. To make a bunch of flat faces look like one big, curvy surface, the edges between them are smoothed.

Just like edges, faces don’t have any thickness. If faces are a lot like pieces of paper, they’re infinitely thin pieces of paper — they don’t have any thickness. To make a thick surface (say, like a 6inchthick wall), you need to use two faces side by side.