A common fraud, called “phishing”, sends messages that appear to be from a bank, shop or auction, giving a link to a fake website and asking you to follow that link and confirm your account details. The fraudsters then use your account details to buy stuff or transfer money out of the account. These fake sites can be hard to spot, so no reputable organization will send a message requesting your confidential information.
Most email readers allow you to view email as plain text. Compare the link in the HTML to the link it claims to point to — the links should be the same. Many blackhats put a legitimate domain and URL in the visual link to disguise the illegitimate domain and URL hidden in the HTML.