Content ID is a YouTube-owned tool that IDs copyrights embedded in content.
Copyright owners (i.e., record companies, publishers and, I assume, film and TV rightsholders) use Content ID to identify, tag, control and monetize content on YouTube.
The Content ID identification process starts when content owners provide Google with metadata and copies of their works. Content ID then uses digital fingerprinting technology to match these works to uses in certain categories of content on YouTube (e.g., matches to user generated videos, but not multichannel networks, which assume responsibility to pay rightsholders).
Not only can the © owner control (with other owners, if applicable) whether to leave the use up or take it down, but also it can choose to monetize (via advertising and/or subscriber revenue). Some record companies get 35% and music publishers get 15% of ad revenue for user generated content. (Big © owners can negotiate directly with YouTube, so those minimums and splits are confidential, but the some rates for indies are public, though they do not include the advances or minimums that major rights holders negotiate.)