Well, for starters you get 5 save files, you also get to use the latest unstable version, (indev for short) which has crafting, farming, smelting, finite water (water won't mass produce itself), pictures and many other things. Indev also has multiple extra things that alter the map, Eg:
floating, island, inland, flat,
original, hell, paradise, woods,
square, long, deep
bought version is better because you can play on servers and change your skin.
You play it.
You can download them without buying minecraft, you'll just have no use though.
Buying it from Mojang directly.
Yes, but choosing your minecraft username is the last step.
According to the official Minecraft site, you have to purchase the game to play it.
No, there is no additional fee.
Yup.
Registering for Minecraft is free, it just set's you up for buying Minecraft. $26.95 US is how much it cost's.
no
By buying the game or getting the game from people.
Im thinking of buying a imac g5 but I don't know if I can run Minecraft on it
A Minecraft account is 100% free. But buying the full version costs 28.95.