Most scams play upon basic human qualities that everyone has to some degree. Many of these qualities are not very flattering. They include traits like fear, vanity and greed. Con artists have leveraged these traits for hundreds of years -- play upon a person's greed and you can convince them up is down and cold is hot.
That also means that most online scams have a few clear indicators. If you receive a message or visit a site that says you are in danger unless you download a certain application, that's a red flag for a scam. The message is playing upon your fear. Of course, you don't want to have your computer overrun with viruses. But often these applications are masking a virus or other form of malware. Always view these messages with skepticism and caution. Research any application before you download and install it.
Most of us would like to think we're not vain creatures. But imagine you're on a social networking site and you receive a message containing a hyperlink that says something like, "You won't believe how great you look in this video!" Most of us would be tempted to follow the link and watch the video, particularly if we were worried it might be embarrassing. With so many people and organizations on social networking sites, you never know who could be watching.
Scam artists know that people are concerned with their online image. That's why they use messages like the one above to direct people to a bogus video site. In many cases, attempting to play the video prompts a pop-up window to appear. The window claims the user doesn't have the right video driver installed to view the video. It prompts the user to download a driver and then the video will (supposedly) play. However, the driver turns out to be malware in disguise.
Some malware can can do pretty nasty stuff if you install it on your computer. A keylogging application could keep track of every keystroke you make and send that information to a remote scam artist. The scam artist can then comb through your keystrokes and find out private information like user names and passwords to the sites you visit, including banking or shopping sites. Others might give root access of your machine to a remote cracker -- a malicious hacker. The cracker can then control your computer and you might never even notice.