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One of the most popular options is KeepVid, which works with YouTube, Vimeo, and a number of other streaming sites. Downloading videos from streaming sites will most likely violate the site’s Terms of Service. Distributing copyright material without permission is illegal. Copy the entire URL (web address) for the video you want to keep. Paste the address into the field at KeepVid. After you have entered the URL, click the Download button next to the field. Do not click the big green Download button below the text field. This is an advertisement, and will open a bunch of pop-ups. After the video is loaded into KeepVid, you will be provided with several links allowing you to download the video in different formats. MP4 is playable on most devices and computers, while FLV has more limited support. You will be given several quality options as well. Higher quality videos (1080p, 720p) will take more hard disk space than lower quality (480p, 360p), so decide which is more important to you. Once you have decided on your format and quality, right-click the link and select Save As… You can choose to rename the file as well as tell your browser where on your computer you want it downloaded to.

Summary:
Find a video downloading website. Enter the video URL. Select the file format. Download the video.