Above all else, movie download sites are illegal, because they offer free content that should otherwise be paid for. There are some that offer legal content, but the main idea behind these websites is to offer free, illegal content to its users. In the end, it is best that they be avoided.
These sites come in two major forms, and one of those forms is streaming. Streaming videos are played directly on the websites themselves and are downloaded at the same time that they are watched. You sacrifice quality for time and storage space on your computer, however. Since the movie is being downloaded while you watch, it takes a powerful computer to use.These sites come in legal forms, though, which is convenient, but that's because of advertisements.
And by many, I mean many. These pages are covered in advertisements and each one requires special attention by your computer, taking away processing power. Streaming videos require a lot of your computers attention as it is, so these ads only make it worse. It all adds to the fact that you'll get a much lower quality video out of it, and so is generally not the form of movie downloading that most people use. The other tends to be more convenient for the idle person.
The other form of movie downloading is through torrents. A torrent is a file is an information file that enables your computer to connect to others around the world. While downloading, you are taking file information from all of these computers at once, resulting in a digital copy of the movie you have the torrent of. The process is slow, but the overall quality of the movie is significantly better because you'll be watching a movie file on your computer, rather than downloading it while you watch.
If you plan to use this method, you'll need to have a torrent "downloader" that can read these files. They are able to make this connection for you and acquire the movie within them. The programs themselves are guaranteed to slow down your computer because how how processor heavy they are, and so are commonly run during times when you won't be using your computer, like while you're at work or sleeping.
Your best bet for downloading movies is through legal sites that either require membership or are otherwise paid for. These sites, since they are paid for, offer a greater deal of movies and at much higher quality. Above all else, these sites are legal, meaning there are no criminal consequences to using them. It may seem easier to not pay for movies, but for what it's worth, these are safer.
How you obtain or watch these movies is the same as the illegal versions that are out there. You can find sites that stream movies or offer download-able movie files. Both forms are paid for either by advertising or by mandatory memberships that have dues. The price for these memberships are generally only a few dollars a year.
It's no question that movie download sites are incredibly convenient, and the prospect of being free is enticing. Their convenience is worth the investment, but being free is not. Look at it this way: by paying a minute amount of money, you are able to view endless video content as long and often as you want. If you get caught downloading illegally, you'll pay thousands in fines, so it's infinitely more expensive.