Video sharing technologies are among the most recent Web 2.0 technologies. Before the availability of sites like YouTube, it was really difficult to upload video to a third-party server and it was even more difficult to be able to play it. Generally, users had to download the video first and then open it in an appropriate video player. Perhaps one of the main reasons for the lack of services like YouTube was that video files take a lot of space. Indeed before YouTube was created in 2005, most email services like Yahoo or Hotmail were given just about 500MB per free user account. Thus, allowing uploading as many videos as wanted was very welcome by most users. Providing an easy to use interface to upload videos and being able to display the videos right into the web browser as soon as they have been uploaded was a plus which contributed to the success of services like YouTube.
Shared online video services can be used in education in many ways: (a) an instructor/teacher can record herself explaining a specific topic; (b) an instructor can search for educational videos related to the topic being covered and show them to the students; (c) students can create videos either individually or as a team for their peers to watch and critique; (d) instructors can use the videos as anchors for instruction (Bonk, 2008), showing the video at the beginning of the class and having the students reflecting upon it.
A potential disadvantage of using YouTube as part of a class is that it includes ANY type of content, not only educational. When playing a video, YouTube shows a list of other related videos which might not be educational at all and students might be tempted to watch those videos, getting distracted from the topic being covered. A viable alternative is using shared online video services like TeacherTube in which all videos are supposed to be educational.
References: Bonk, C. J. (2008, March). YouTube anchors and enders: The use of shared online video content as a macrocontext for learning. Paper presented at the American Educational Research Association (AERA) 2008 Annual Meeting, New York, NY. http://www.publicationshare.com/SFX7EED.pdf