It depends on the person, really. If you feel pervasively tired during the day, you probably need more sleep. I personally feel my best when I’ve had 9–10 hours of sleep, but most of the time I get about 7 or 8. Your lifestyle, including what kind of work you do, how active you are, etc., is a factor in this too.
I once took an online test about this, and for fun I put in some false info where I said I was getting only 4–5 hours per night or something. Based on my other answers, the results said I was probably getting around the right amount of sleep!
As for the effects… WebMD has a good site describing how sleep deprivation can negatively affect your health.