Telemarketing is the sales practice of cold-calling people to try to sell things. This is an almost universally irritating practice (see the angst captured in
How to torture a telemarketer for instance - oh, the
zeitgeist cries out in pain!) - since telemarketers usually call in the evening at private residences, and take advantage of the normal politeness telephone
etiquette demands - it's rude to hang up, so many people stay on the line, a
captive audience. Telephone
solicitation is also a hotbed of
fraud - it's easy to represent yourself as anything you want on the phone. It's probably a good idea to check with the
Better Business Bureau before buying /anything/ over the phone, unless it's a reputable company and you called them - not they called you. Thousands of people get burned every year by telephone fraud.
If you are irritated by telemarketers, the best solution is probably not to harass the caller - the poor grunts who work at call centres are poorly paid and generally have no power. What is more effective is politely asking to be removed from their call list - in more jurisdictions, they are legally required to do so - and then hanging up. If they call again, report them to the appropriate authority.