Scams and Scares
The classic scam is the 'Nigerian' (from its most prolific creators). An unsolicited email identifies you as the beneficiary of some massive sum of money, and all you need to do is to contact the writer to accept it. The usual process for relieving you of YOUR money is either to obtain your banking details and withdraw from it, or to ask that you pay certain fees to release the money (which fees promptly vanish with no recourse).
Some Nigerian scams make you complicit in felony - the author has chosen you because the money is banked in (what a coincidence) YOUR family name, so you can claim it (but it is NOT yours).
Related scams pull at your heartstrings - you are asked to send money to some unfortunate child - or your heart - Natasha/Romeo is sure (s)he could fall in love with you and marry you if you sent the money for a plane ticket.
A common scam, called phishing, obtains your banking details without offering you money - an email, notionally from you bank, invites you to login to on-line banking and fix up some error. In fact you are linked to a simulation of the bank's site, and your login details are captured. Your bank will NEVER send you unsolicited emails. Note that if you hover your mouse over a link the real target address will pop up; this makes it easy to see that you are being tricked.
Phishing scams also offer you updates to current software - but the update installs malware instead. The software on your computer will update itself if you choose the right command (typically Help→Update).
Other phishing scams target your email login - you might think that it is of little value, but a valid email login sells on the market for upwards of 50c, and can then be used to launch spam messages in your name.
Variations on phishing techniques include an invitation to open an attachment to the email. Be very careful of opening attachments. Phishing messages can easily be 'sourced' from a friend's email address - so if a friend sends you a message asking you to view an unlikely site or open an unlikely attachment check back that the information really came from that person.
A prolific source of spam (nuisance messages) is the scare message. A friend tells you that there is a new and dangerous virus hidden in any email entitled 'Resignation Of Barack Obama', that it has been described by Microsoft as 'the worst ever', and you should warn all YOUR friends of its existence.
Before forwarding the warning PLEASE verify it by searching (using your preferred search engine such as Google) for "Resignation Of Barack Obama hoax message" (in this example).
This sort of hoax spreads widely and just serves to clog peoples' inboxes. Try not to contribute to the clutter.