If video from youtube is very easy!
1.If it is embedded in the site - run it, then click on the youtube icon and we move on youtube.
2.Then go to the address bar youtube.com/.... and adding "ss", thus to get the address ssyoutube.com/... - here there and go and download.
Also there are programs that download, if you provide them the link to the video on youtube or another popular service. For example, Download Master(westbyte).
If the video is not from YouTube, then in most cases it is also simple to download.
To do this in browsers, such as Firefox, Opera(old) there are plugins detects video and rocking them.
In Chrome, things are even simpler:
1.On the video page, right click anywhere on the page and select in the list "View code" or Ctrl+Shift+i. The video does not run!
2. Go to the tab Network launched video on the page and write the filter this is usually mp4, flv, webm or instead of the DAB on the filter Media. prntscr.com/cz9fba
3. In the list find the file, right-click - Copy - Copy link.
4. Insert the link in any download Manager, same Download Master for example.
Or: open in browser - opens the player also right-click - Save video as.
Download Manager is preferable, since it allows to download in several threads, supports resume, pause, etc.
For example, some sites give a link to a video on a certain IP and for a limited time. Then, if you change the IP(Internet Rabotnitsa), or if a large amount and not manage to download or something, you will need to get a new link and in download properties to specify a new link source.