Who are the people changing their minds?

As a person closely following the presidential race, I am always surprised when the polls change. There are many websites on which you can observe poll figures, as they change by day. In many states, the polls have changed drastically. I’m wondering, “Who are the people changing their minds?”

Everyone I know has known for months which candidate he/she will be supporting on election day. I don’t know a single person who has changed his/her mind. While it certainly can happen, I was under the impression that such a shift in opinion was a rarity and did not reflect the gigantic percentage points that are consistently jumping in each of the polls.

Does anyone know why the polls results change so frequently? Are people really changing their minds, or are different people polled everyday? How many people are polled? How generalizable are the polls to what’s going to happen in the actual election? (I know how statistics work, but there are obviously a lot of other factors involved here.) Also, why do different polls produce such different results?