Example: If you want to play a video file in google chrome browser, u need to have a player as plugin in your browser google chrome. Flash player is integrated as plugin in your browser to watch video. If not, u need to download it as plugin. Whenever there is a need of downloading a plugin, thare will be an image type showing which has the link to download. You need to click at that image. The image will look like above image having a bold text sentence.
If you visit an unknown website and ask you to download a plugin, it may able to hack your computer by that website owner and steal your personal datas from your computer. Once your computer is hacked, it can be used from distance through internet by the hacker. But hackers can do this only when you connect your computer to intenet. Moreover virus may be integrated with plugin which will be harmful for your computer.
