Video has always been a part of education. Teachers often use movies in the classroom whenever the content of a movie or documentary could serve as a supplement to classroom instruction.
While up until the Internet, video consisted mostly of VHS tapes or DVD videos, with the advent of the Internet and YouTube, teachers have a much larger toolset they can use to apply informational YouTube videos in the classroom.
Videos to Watch in Class
At a very basic level, you can simply hook up a computer to an overhead projector, connect the computer to the Internet, and then play YouTube videos in the classroom. This replaces the need for a television in the classroom - and in full-screen mode you can project a video in a classroom without losing much of the quality at all.
There are lots of great online Youtube lessons that are perfect for the classroom environment, such as this science lesson about DNA produced by the
Innocence Project.

DNA Science Lesson
There are lots of lessons available covering a range of subject matter. For example, an English teacher covering vocabulary and grammar could play this lesson
created by Jennifer from Massachusetts, titled "Outdoors English Vocabulary".

English Vocabulary Lesson
One subject where YouTube really excels is history. You will find a very large selection of documentaries and lessons about many historical matters.

History Lessons
For example, The Holocaust Museum offers documentaries and classroom lessons, such as one with classroom instruction including teachers and students interacting and talking about the subject.
Of course, using YouTube for the classroom goes far beyond just playing movies in the classroom. You can also use YouTube to encourage interaction by students. For example, as an assignment, you can have students include videos as part of a research project. They can upload those videos to their YouTube account.

Uploading videos
Of course, it isn't advisable to make those project videos available to the entire public, only to the teacher and potentially classmates as well. Students can prevent videos from being displayed publicly by clicking on the button to Edit the channel, then clicking on the "Info and Settings" tab, and finally deselecting the "Make my channel visible on Youtube" checkbox.

Setting Correct Privacy on YouTube Videos
Also, keep in mind that while YouTube videos are great inside the classroom, they are just as effective outside the classroom. Teachers could create a special account for a class, and then generate a "playlist" of videos that students can access from home.

Typing Skills
Students simply need to go to the teacher's YouTube account and then click on the playlist title to access the video library that you've set up for them. These could be videos that may help with classroom assignments, or just videos students could watch and then write an essay about for extra credit.

List of Playlist Videos
When Students open the playlist, they can select to watch any of the individual videos that you've set up within the list itself. Playlists are a very useful tool that you can use to organize groups of videos that may supplement classroom instruction.
Another very cool use of YouTube videos is presenting step-by-step instructions for projects inside of the classroom. This could be anything from building a model volcano in Science class to how to properly structure a story in English class.

Step-by-Step Instructional videos
Projecting these videos on a big screen during the classroom activity can free up the teacher to assist individual students while the video provides the instructions.
Another very useful way to use YouTube videos in the classroom is as a stand-in when the teacher can't be present. This means that during those days when a teacher takes a sick-day and a substitute teacher stands in, a Playlist can be used as a method to teach the class without actually being there.

Using a Playlist Series
You can set up a series of videos that will last up to the entire length of a class. The videos may offer lessons that align with what you've been teaching during this school term. All the substitute teacher needs to do is open your playlist and press the "Play All" button. Each video will play subsequently one after the other.
Best of all, if you want a comprehensive solution customized by YouTube itself, then you'll want to take a very close look at YouTube for Schools, an area of the YouTube site created specifically for using YouTube inside the classroom

YouTube for Schools
There, you'll find all educational videos, as well as an easy way to set up specific videos that can only be accessed from within your school's network, and nowhere else.
As you can see, YouTube videos can be an excellent tool for the classroom. By creatively using the many tools and content that YouTube offers, you can enhance the classroom experience for all of your students.