Here is the Wikipedia definition of Permission Marketing:
Seth Godin offers four free chapters to his book on Permission Marketing here. I'm also participating in (an at present closed) group he started at Triiiibes.com.
Permission marketing is a term used in e-marketing. Marketers will ask permission before they send advertisements to prospective customers. It is used by some Internet marketers, email marketers, and telephone marketers. It requires that people first "opt-in", rather than allowing people to "opt-out" only after the advertisements have been sent.
Marketers feel that this is a more efficient use of their resources because advertisements are only sent to people that are actually interested in the product. This is one technique used by marketers that have a personal marketing orientation. They feel that marketing should be done on a one-to-one basis rather than using broad aggregated concepts like market segment or target market.
Permission based marketing is believed to have been developed by Seth Godin, a well-established international marketing guru at the turn of the century. A key element of "permission" based marketing is that you are in essence, purchasing someone's time and getting their "attention" which has become increasingly valuable in what may be termed the 90-Second Economy.
Clearly, the highest degree of Permission Marketing occurs when you enjoy great relationships with satisfied clients, who welcome further marketing communications from your organization. This should not be confused with pestering 'follow ups' -- I'm referring to the situations when you treat your clients so well they want to do business with you, and your follow up materials and offers provide real value in themselves, not just as come-ons to purchase more stuff.
Of course Permission Marketing is at the opposite end of the marketing frame of reference from hard-rock cold calling, telemarketing and canvassing, where the prospective client (or not) has no choice but to respond to the marketing inquiry, if only to (in anger) slam the door or phone!
In practice, we find a polarity in marketing between Permission Marketing and the other extreme -- intrusive marketing -- with less room for the other formats, primarily advertising, where you hope to attract attention through paid messages and cause people to contact you. In general, I expect you are in the best position if you can rely on Permission Marketing for virtually all of your marketing initiatives, but respect that in some cases you simply don't have enough stuff in the pipeline -- and you need to reach out and pull the business in. Then, the other, intrusive, alternatives, have a valid place in the picture.