Don't take photos of people in public unless you get signed permission. Many people are concerned about their photos showing up on the internet, and its a valid concern. If there are children with them, you could end up in trouble. In some countries, you will end up in jail by photographing people without permission. Claiming that you were not actually photographing anyone, is not going to be believable, if a person sees a camera pointed their way or toward their children. As more and more so called street photographers take photos of people and children without their permission and post them on the internet, we will move closer and closer to totally restricting the use of cameras in public places.[\soapbox]
Totally untrue - except for the "in some countries" part. I'm sure there are some countries that prohibit such activity. Countries like North Korea and Iran for instance. Here in the USA, if its seen from a public place, its ok to shoot. There is no expectation of privacy, whatsoever, in public. The only "caution" is what is, and what is not, public. For instance, a public park, is public. A theme park, is not public - its private, with paid admission. I got ejected from the Palm Beach County Fair one year for taking pictures. That is, the Fairgrounds is public, but the event is private, held by private company that rents the public Fairgrounds for their private event! And so it said, on the back of my ticket stub, in fine print "... no commercial photography..." amid the rest of the clutter in about 2pt type. The cops said because I was shooting with medium format equipment, I was considered a commercial photographer. I didn't push the issue... left quietly, as I was pretty much done for the evening anyway.
My wife and I have been for two months in Iran and only once, near Persepolis, I was politely ask by the police not to point eastward as there was a military post. At Teheran, Qazbin, Isfahan, Kerman, Shiraz, Bam, Mashhad, Tabriz, Ardabil had absolutely no problems taking people photos. Furthermore, we got invited as guests of honor to a wedding in Mashad, to eat on many occasions, and two times the taxi driver refused to be paid. I encountered problems only in Quom but I still wonder if it was due to me being in a different mood or the people. Iran is, by far, the most welcoming country I have been in. (I'm european and only speak english and spanish).
I've travelled extensively and intensively for the last 18 years in Africa, Asia, Europe, North Central and South America and, in my personal experience, the places where you are more likely to get in trouble are: taking photos of the indigenous people of Central America and on many places on the US's midwest.
But this is only my experience.
I try to be polite, don´t feel guilty if I take photos without permission and always smile. When confronted, if needed, I apologize.