From Jakob Nielsen, "Users don't just confuse search fields; many less-techy users don't understand the differences between many other common features:
- Operating-system windows vs. browser windows
- A window vs. an application,
- Icons vs. applications,
- Browser commands vs. native commands in a Web-based app
- Local vs. remote infoDifferent passwords and log-in options (users often log in to other websites as if they were logging in to their email)"
I can attest to all points after watching my mom, dad and non-tech friends use a computer. For my parents, I believe it's because they never form a functioning concept around "the computer". I've tried to explain directories vs. folders vs. files a 100 times to my dad. It never translated into a working concept in his brain. So I stopped explaining and now use Logmein to take control and get his brain out of the equation.
Not understanding the "mental model" (as Nielsen puts it) or "conceptual aptitude" (as I just now put it) of the user is the numero uno mistake of web design.
I hate agreeing with Jakob Nielsen, but I will admit he right most of the time. I just wish he could frame his idea better than the typical "84 Rules You Should Follow".