The iPhones and iPads have full blown web browsers, so normal HTML-web sites look the same on these devices as on any other computer. But, they don't support Adobe Flash, so if a site uses Flash, those parts will just be empty, unless the site handles it specifically.
And also since the screens are smaller than normal computer screens, things will of course be smaller. And touch screen navigation instead of a mouse might not let you see things that you see when you are hovering over an object on the page.
The iPhones and iPads have full blown web browsers, so normal HTML-web sites look the same on these devices as on any other computer.
Well, almost. The browsers, it seems, aren't quite standards-compliant, so small things can turn out differently.
The small display size can also create havoc with things like control bar targets that are fairly small to begin with. A fat-fingered user can have trouble navigating reliably ("No, I wanted the 'next' button, not the 'close' button!).