As much I have understood from the Terms of Service, it's not a straight yes or no. Basically, Youtube doesn't have a publicly available API or service that would allow users to download the videos. Of course, you can download back the videos that you had originally uploaded. But who would do that anyway?
However, there are workarounds and tricks with which you can actually download any video from the website. There are many softwares/add-ons that easily do this job. Here is a snapshot from the published "Terms of Service".
You shall not download any Content unless you see a “download” or similar link displayed by YouTube on the Service for that Content. You shall not copy, reproduce, make available online or electronically transmit, publish, adapt, distribute, transmit, broadcast, display, sell, license, or otherwise exploit any Content for any other purposes without the prior written consent of YouTube or the respective licensors of the Content. YouTube and its licensors reserve all rights not expressly granted in and to the Service and the Content.
|From Youtube Terms of Service|
It says, that you cannot exploit the content/content-owners by selling the videos or re-distributing it, thereby, making a profit against your sales. That's obviously illegal for any kind of content, unless of course, the license makes that content freely re-distributable. So, if you download the videos using those workarounds and tricks, only for your personal use (ex, offline viewing), you probably aren't breaking any rules. What about those add-ons/softwares that do this? Well, it's not illegal for them, since they are just distributing the software. So, they are off the hook.
For personal use, it seems ok to download videos off from Youtube. So? let's see how hard or difficult it is to get this working.Actually, it's quite simple.
Step 1: You should have the VIDEO ID of the video that you want to download.
Step 2: You need to make a call to this api, to get the details for that video. The fmt parameter is for getting the specified format of the videos. See the "Quality and Codecs" section on this page on Wikipedia.
URL => http://www.youtube.com/get_video_info?video_id=VIDEO_ID&fmt=6Step 3: Process the response. You will get a plain string response. That response contains all the direct URLs to various formats of the video. Look for the key "url_encoded_fmt_stream_map" and the value for this key is what you need from this response. You will need to filter out all the URLs from here, and get hold of the URL to the format you want to download. Here are some sample URLs.
http://o-o.preferred.bharti-bom1.v8.lscache7.c.youtube.com/videoplayback?sparams=id,expire,ip,ipbits,itag,source,ratebypass,cp&fexp=904510,914501,908302,902315,916201,905267&itag=45&ip=126.96.36.199&signature=095A9503E49931B0B849257048E73EB7388F515A.C13F4074ED92A82DF94BBD73E808E917369C224D&sver=3&ratebypass=yes&source=youtube&expire=1326412828&key=yt1&ipbits=8&cp=U0hRS1RMUF9HUkNOMV9MRlRJOjZ2TTNoOG9ialZD&id=c418f3a4b0f1b751&quality=hd720&fallback_host=tc.v8.cache7.c.youtube.com&type=video/webm; codecs="vp8.0, vorbis"&itag=45
http://o-o.preferred.bharti-bom1.v11.lscache3.c.youtube.com/videoplayback?sparams=id,expire,ip,ipbits,itag,source,ratebypass,cp&fexp=904510,914501,908302,902315,916201,905267&itag=22&ip=188.8.131.52&signature=2C46F096073FDACEFD3B4895EDBC3CA1162682AD.3BCAE9E76BBE98D3FB687AF16ECC595E35AD8173&sver=3&ratebypass=yes&source=youtube&expire=1326412828&key=yt1&ipbits=8&cp=U0hRS1RMUF9HUkNOMV9MRlRJOjZ2TTNoOG9ialZD&id=c418f3a4b0f1b751&quality=hd720&fallback_host=tc.v11.cache3.c.youtube.com&type=video/mp4; codecs="avc1.64001F, mp4a.40.2"&itag=22If you notice the URLs, there is a type parameter, where you can determine which URL is for which format (type=video/mp4, type=video/webm etc).
Step 4: To finally be able to download the video, you need to strip off a few values from these URLs, just to make sure your calls don't fail due to long URLs. What I have noticed is that if you strip off (everything after the quality param) the last parts of the URLs, everything's just fine. So, the final URL would be something like:
Step 5: Save the file with the proper extension. That's it. You are done.
If you ask how I got to know about this trick? Well, everything's already out there on the web. A lot of people have already blogged about it before. But, I had to dig for it for almost 2 days. So, I hope someone finds it useful. So, if you want to make your own Youtube Downloader, now you know "How to Download Youtube Videos programmatically".
Note: This method might stop working as and when Google/Youtube blocks this loophole. In the past, Youtube has been known to block a few other workarounds that had existed.