From BlenderWiki

The (E) and (N)

Poles in general are not good for your Organic models however, you cannot avoid the 5 and 3 edges poles and instead of being afraid of them why not try to understand them?
Many beginners cannot advance to the next level because of this blockage about poles and so they tend to fear them and then Topology becomes difficult and they fear that too. The 5 and 3 edges poles are very special poles, 6 and beyond are not special so you can ignore them completely and since there are only two special poles we're going to give them names.

The E Pole ("extrude" pole, 5 edges)

E pole

The E pole is actually an Extrude Pole (E for short). When you extrude a Quad you will get 4 Es and 4 Ns.

When you extrude for the mouth, eye and ear you'll get 4 Es and 4 Ns each. When you extrude for the arm/leg you also get Es and Ns.

The N Pole ("nose" pole, 3 edges)

N pole

When you model a nose you'll get this 3 edges pole and there is no way you can remove it because if you do then that nose will not look like a nose and so it was meant to be there (Keep that in mind).

I call this 3 edges pole The Nose-Pole (N for short).

The Nose is a very special case in that you get “E” and “N” next to each other, I call this the “EN” case. This “EN” situation will surfaces once you get into detailing like the nose here. If you remove this “EN” you will remove the detail for the nose and in the image above I have separated E and N with a Loopcut (more on this later).

When you extrude a POLY that has N pole as one of its corners, you will convert that N into nothing (no pole)!

Now why is the talk on Poles important? Poles control how things flow in your topology, have a look at the image below.

The talk on poles will helps us to understand flows and smooth/bumpiness of our meshes.

Next: Flows