Offers4U is an annoying ad-inserting thing for browsers that pops up large annoying "offer suggestions" and other junk when browsing the web (which also implies that it is taking a bit too close a look at your browsing activities..).
There is plenty of information about how to get rid of it on Windows, however there wasn't any clear information on how to get rid of it from Chrome on a Mac.
After a little exploration I found that it is part of the seemingly-unrelated "SafeFrom.Net helper" that claims only to be a tool to make it easier to download videos from youtube and other streaming sites.
To get rid of it, open chrome://extensions/ in chrome, and then disable the SaveFrom.net helper extension. I guess if you still want to be able download from youtube and other places (e.g., if you lost the original of a video you uploaded some time ago), then you can turn the extension on, open a new tab, and then disable it again after completing the download.