On the spam has already been mentioned. In Basically – this is illegal advertising. The purpose of a spammer – you send it to the desired site, or download an infected file (if you find an attachment with a double extension such as *. jpg.exe or *. rar.exe, do not try to open it, at best ask money), or run on a computer the actions that would damage security. Look at who the letter. If your address is specified explicitly, the decision is yours, and if the address bar, then remove the pile of incomprehensible through the 'This is spam' not reading. Infected web-page.
On such resources you can slip a Trojan disguised as useful programs from (flash-player for the browser, any application), running that you do give the command to execution of malicious code or virus, which uses security holes in your system. Methods of struggle are as follows: attention, timely updates of virus databases and software, work with limited account. There is a special software that creates a sealed environment for the work (including the free, such as Sandboxie – 'sandbox'), but that's a different story. Fraudulent sites – common 'scam' divorce for money. WOT helps in most cases.
Look for information about the owners, the availability of addresses, e-mail and phone for feedback. If you have something to try to sell, and that information on No site is better to refrain from buying. Phishing sites – sites that are almost exact copy of the official Internet resource. Created to retrieve data for your accounts. Especially dangerous if you do online payment or funds transfer. The recommendations are simple: always look on the address bar, the address must exactly match the original, do not go to suspicious links, do not leave their data on unproven resource, never give out your password from your account, and even more so – the PIN code of your credit card. Before you go to the link, read what you offer. "Free cheese – in a mousetrap." More often vice versa. Came to the site and think, 'How and why did I get here? ". Why, think later – a fraudulent website has already performed its task.