Basically, you can do whatever you want.
An STD can be just that - a notice (postcard/magnet/etc.) telling people the date (and maybe location) that you'll be getting married. Or it can be much more elaborate. For me, my STD was done passport-style and had 12 pages of information. Everything from the date, to the resort info, dress code, TA contact info plus website info. BUT, I sent mine out 6 months prior to the wedding. If you've got a longer period of time, just a postcard should be plenty if you want to go that route. I'd definitely set up your website first, because people will want the info, but you can include the address in the STD and they can go from there.
Invites, and who gets them are again, totally your call. For me, everyone who got an STD will get an invite. Even if they've declined. But that's only 35 or so, and not that big of a deal. If you're sending out 100, it's more significant both in terms of work, and in terms of cost.
On the website - I have one thru wedding window and I LOVE it. It's so easy to do. And you can put anything and everything on it. Photos, information, stories, events, RSVPs, etc.
Good luck!!