Spammers are using a feature in Chrome to fill victims' browsers with unwanted adverts.
Chrome has a feature which silently automatically updates everything including extensions, meaning that it is up to the user to decide if the owner of an extension is trustworthy.
But Chrome users are finding that ownership of a Chrome extension can be transferred to another party, and users are never informed when that happens.
Malware and adware vendors are apparently showing up at the doors of extension authors, looking to buy their extensions. Once the deal is done and the ownership of the extension is transferred, the new owners can issue an ad-filled update over Chrome's update service, which sends the adware out to every user of that extension.
Google has said that it is aware of the problem and said that Chrome's extension policy is due to change in June 2014. The new policy will require extensions to serve a single purpose which will mean the end of the adverts.
If your machine is full of ads, you might want to disable your extensions, particularly the more obscure ones. It is just another way that advertising people screw up life for the rest of us and are part of the reason we can't have nice things.