How are people selected to take political polls?

There are some market research companies who actively send people out into the communities to ask x amount of people of their opinion. Some on the street, some by phone. There are also communities in this country which are thought of being representative of the general population. Things like that. Polls are polls and as such not always very reliable. Although the pollsters have become more sophisticated and have developed greatly sophisticated statistical programs to analyze the data provided to them. Still, can't always go by it, especially, when it is close.