You send out emails to various people from a list. Your email would include your products and services and a link to your site and such. It is another way of advertising, which in my opinion does not really work (as far as I have tried it).
Many companies still do it, but I guess you have to have a really good sample of emails. If you have people on the list, who are not interested in your niche; then there is no point in sending out the email to them. For example; if your company sells auto parts, you will need to target mechanics, car dealers and car repair shops. You would have much more luck with them compared to regular people off the street.
Email lists with a good turnover, will cost a lot though.