Yes. E.G. there is classic, classic _1, infdev, indev, 1.3, 1.4, 1.5, 1.5_01, 1.6, 1.6.1, 1.6.6, and 1.7.3.
The different minecraft versions are incompatible.
Good seeds vary between different Minecraft versions.
No. You cannot connect with other players with other versions.
If you use MultiMC you can have multiple copies of Minecraft installed on one computer, with different versions and mods.
If you want to play Minecraft on a different version than the most recent, then you will need to look online for a Minecraft server list and make sure to include the version number that you are trying to play on.
Mojang does not sell previous versions of Minecraft. If you have a Minecraft account, download a mod which allows you to play previous versions of the game.
You can download "Minecraft Nostalgia" to downgrade versions of minecraft
Not currently but in the future you will be able to.
Windows/Mac versions are $26.95 US, slightly more or less in different country's.
The newest version of Minecraft as of October 4, 2015 is 1.8.8, but the newest snapshot of Minecraft is 15w40b
There is no time machine in minecraft. The "Time Machine" is a nickname for the Minecraft Launcher, the startup of the game. You can access old versions of Minecraft.
You can't. You can, however, mine it in the snapshot versions of Minecraft 1.5