When posting youtube videos at OC3D most users simply copy and paste the URL. This works to some degree, your video does show up embedded in the post. There are some caveats, however. The aspect ratio can be distorted and it can actually break the forum layout.
This is where the youtube tags come in. They allow youtube videos to be embedded in a post, but without breaking the forum layout and without changing the aspect ratio of the video itself.
Let's look at the url for a typical youtube video. The following is Tom's review of the MSI GTX 780 Lightning.
The important bit we need to look at is the string after the v= in the URL. In this case it's 9jcCuREk5dY. If we copy only that section of the URL and place it in between the youtube tags built into the forum, the video will be embedded and shown properly.
Below is an example of the code needed:
Here is the resulting video embedded into this post: