Videos uploaded to YouTube or Vimeo can be inserted into Articles easily. Simply navigate to the desired video on either YouTube or Vimeo, copy it's embed code and then paste that into your Article where you would like the video player to display. The result will be an inline video player within your Article.
To get an embed code from YouTube:
Navigate to the video on YouTube
Click the "Share" button under the video
Select "Embed"
Copy the embed code using the "Copy" button on the bottom right of the window shown
To get an embed code from Vimeo:
Navigate to the video on Vimeo
Click the "Share" button under the video
Click into the Embed code text box and copy the full embed code