Trademark bidding refers to the practice of buying trademarks as keywords on Google AdWords and other sponsored links platforms.
Trademark bidding may be done by the legitimate owner of a trademark, by competition or by some affiliates.
To bid on his own trademark is a matter of debate among AdWords consultants and users as it often seen as buying a free potential traffic (the brand has the more often the first organic ranks on his brand keyword). Nevertheless, it may have some benefits to let less space to competitors.
Despite legal risks, some competitors do trademark bidding.
Affiliates may also do trademark bidding. According to affiliate contract, it may or not breach the contract (for more details on this topic, see trademark poaching)