Barter marketing, attempting a definition

Barter marketing is often a cheap, original and effective way of reaching your target group as a marketeer.
You need to find a company with a similar/idential target group as yours. Then you strike a deal where you will promote the other company to your clients/members. The other company will then promote you in return to their clients/members. If done well, you’re both better off.
No or little cash is exchanged. And the marketing is usually quite credible. Your clients would normally expect you to only promote somebody else if this other business/organisation is any good and relevant for them.
In short, barter marketing is smart marketing.