Why are election polls so far apart??

Question

One poll, the CBS new york times poll is showing OBama ahead by 8. The CNN poll and most other polls are showing ahead by 1 or 2 points at the most. The Zogby poll is showing mccain/palin ahead by 3. What's going on?? This week is the worst. At least all the polls in the last 2 weeks were somewhat close to each other. I know that polls don't mean anything but still, which is close to the right??

Answer 1

About a 1000 people are polled each time, which is supposed to represent the opinions of 300 million Americans.  Obviously, with such a small sample, there will be a margin of error.  Most of the polls state that their margins of error are around +/- 3%.

Answer 2

There are usually differences is the qualifications of the people they poll.

Registered voters will have different numbers than likely voters.

Democrats or Republicans only polls will be different than those with Independents.