It is multiple email messages sent to addresses that have not requested to be put on the spammers mailing list. Creating mailing lists is not prohibited, but putting people s addresses on mailing lists where they have not requested it is.
Companies use software to trawl through newsgroup postings and web pages to gather email addresses.
These lists are then used directly or sold on. For users it costs time and money to download the UCE mail and makes using the Internet more difficult. The majority of Spam is either chain letters and get rich quick schemes. For an ISP it can hinder their ability to handle users email efficiently, plus they have to ensure strict security measures are in place so that UCE organisations cannot hack in. In summary UCE companies can steal resources which effect both users and ISP s at little cost to them and in most cases little commercial benefit.