1) What is "shared" hosting?

I am pretty sure its when you have more than on website sharing the same server space and of course each one would have to have its own domain.

2) What's a "mySQL Database? Does that mean I could set up ecommerce on my site?

SQL, PHP and XML are all database related they are used to store and process data over the net so you can have ecommerce sites, messageboards, online scheduling, interative price list, PHP however is server based and you need hosting that supports Linux to have it but, Its mainly used for messageboards.

3) If I'm creating the pages on a Mac, do I need to look for any special support from my host?

HTML, Javascript and all of the other web programing languages are universal it will show up the same on a mac as it does on a PC computer. The only time you run it to problems is with video and images, for example if you put up video that is a windows media file depending on what software the user has on his mac you may or may not be able to view the video. But most of the web programs you buy can be bought for mac and pc so it doesn't matter

4) If I'm locked into Register.com for email for 5 years, can I still use another host for email?

Yes, POP3 emails as far as I know are set up through micrsoft Outlook I am doing my website and that was how I was told to set it up. You can add more POP3 emails but and have them through different host if you wanted to

5) Why might I want unlimited FTP Accounts, or unlimited subdomains?

Subdomains are sites that are connected the main website that you have so if you wanted to have multiple sites on one server without having to pay 20 bucks everytime you want create a new site its a very good thing to have, FTP stand for File Transfer Protocol and what that is, is simply the images, html, video and whatever else you upload onto the server so it is available on the web. Unlimited FTP accounts is good for having more than one webstie on a single server if you are just doing 1 site all of those extra options are a waist of money.

6) What is "Fantastico"? I have no Idea be more specific.

7) What is "PHPMyAdmin"? I am not sure what exactly you are asking but Having administrative features on a messageboard allows to add take away post and all of those extra little features that a moderator has. You are going to have to be a little more specific with your question.