It is certainly a form of extreme torture, a horrible way to die. You may be glad to hear, therefore, that Iran appears to have suspended the use of stoning.
More generally, stoning people to death for adultery is still legal in quite a few places. Most of those killed that way are women. Why? Here's one possible answer:
Women constitute nearly all those condemned to death by stoning. Why? Because discriminatory laws and customs almost always assign more guilt to women than to men in any manner of action that is seen as violating 'norms' of sexual behaviour, especially any instance of alleged sexual relations outside marriage (zina). Men are entitled to marry more than one woman and can use this justification for sex outside marriage. They are also more mobile and can more easily escape punishment.
This quote comes from the stop-stoning.org. The same page discusses honor killings:
According to the UN Special Rapporteur on Extrajudicial, Summary and Arbitrary Executions, so-called 'honour killings' (or rather, dishonourable killings of women) have occurred in countries as diverse as Bangladesh, Brazil, Ecuador, Egypt, India, Israel, Italy, Jordan, Morocco, Pakistan, Sweden, Turkey, Uganda and the United Kingdom.
That is a very odd way of putting it, that "have occurred in countries as diverse as." It's not the case that suddenly Italians or Swedes, say, have decided to adopt the custom of honor killings. The custom was imported to those countries with immigrants who already had it, and we should be able to state that.
The website is probably just trying to make the point that honor killings should be of concern to people all over the world. But I dislike the veils drawn over the cultural nature of the problem.