Orthodox Jews and many Conservative Jews keep kosher. This is a series of dietary laws as laid out in the Torah and explicated in the Talmud and later works. Major rules include prohibitions on the consumption of certain types of animals, a prohibition on mixing meat and milk, and requirements for how animals must be slaughtered in order for the meat to be kosher. Many Orthodox Jews will only consume food that has been specifically supervised to ensure that no non-Kosher contaminants have entered the food. Such supervision is marked on food packaging with small labels indicating what organization supervised the food. This symbol is called a hechsher (plural hechsherim). For example, in the United States if you look on many foods in the United States you will see a small U inside a circle on the packaging. This symbol indicates that the food is under the kashrut supervision of an organizaton called the Orthodox Union.
Why am I thinking about kashrut supervision today?
Wolfish Musings talks about a new milk being sold in New York and New Jersey that is aimed at the ultra-Orthodox population has no less than four hechserim. The vast majority of Orthodox Jews have no issue with food that has a single hechsher. Indeed, it is generally difficult to find a food with multiple hechsherim on it. Moreover, while arguments do exist in the Orthodox community over which hechsherim are really trustworthy enough, most of those arguments are about the hechsherim for meat products. The laws concerning milk products are much simpler than the laws for meat. Ultra-Orthodoxy seems to be about trying to find new ways to be strict that no one previously has ever worried about. Indeed the motto of the milk company is "Machmirim bnei Machmirim" which translates as "The strict who are children of the strict." If anyone can give an explanation for why someone would think that four different hechsherim would be at all useful or halachically better I'd like to hear it.