Brandjacking in AdWords is simply the process of bidding on competitor's brand terms and then showing your own ad. Here's an example of Freelancer attempting to brandjack oDesk. Notice that Freelancer has the second ad, and is aiming to take the searcher over to freelancer.com.au even though they were specifically searching for oDesk.
The practice is allowed by Google in most countries - especially with regard to bidding on search terms. However, companies can make requests/complaints about the actual content of the ad copy if the terms are trademarked. In the example above oDesk would have a strong complaint case against the Freelancer ad copy, especially since the capital 'D' in 'oDesk' indicates the ad copy was purposefully written, and is not just the result of keyword insertion.
This is a good reason why you should usually bid on your own brand terms - at least if a competitor is attempting to brandjack your terms, you'll still (likely) appear above them.
Here's a recent example of brandjacking - with a counter by the owning brand (via Philip Gamble on Twitter):
