Digital watermarking, the technology of embedding special information into multimedia data, is a topic that has recently gained increasing attention all over the world. The watermark of digital images, audio, video, and other media products in general has been proposed for resolving copyright ownership and verifying the integrity of content. The characteristics and applications of watermark techniques are first introduced, and then the basic concepts and evaluation criteria are expatiated. For further understanding, the watermark techniques from the various aspects are classified and some conventional watermark techniques and algorithms are analyzed in detail. At the same time, their security and performance are compared. Finally, the possible research direction of digital watermark technology are pointed out.