Not necessarily.
Online, relying on a Web site's promise of security can disappoint you -- even if the firm did nothing wrong. There are lots of hackers and phishers on the Web who don't hesitate to lie, and innocent users are regularly ensnared in digital honeypots used to commit fraud and deception.
While it's virtually impossible to ever be truly "secure" in the era of always-on connections, there are plenty of authentication technologies that allow a user to be pretty sure who they're dealing with.
SSL, which stands for Secure Sockets Layer, is a widely used communications protocol that allows users to communicate with websites via encrypted links that aren't easily hacked or spoofed. The ubiquitous "lock icon" is a good indicator that a Web site is both secure and genuine. SSL's website has a very informative FAQ explaining how users can detect common tricks employed by unscrupulous websites.
Third-party ratings Web sites are also invaluable, especially when dealing with unfamiliar vendors. Resellerratings.com is one such resource, with hundreds of thousands of customer reviews of just about every major online store.