Minecraft can be played online or offline. When you play online, you need Internet connection and you can play with other people. When you play offline, you don't need Internet connection but you can only play by yourself.
Yes you need internet to play online multiplayer
To download it from the website you need internet connection of course but once it has been downloaded you can play in singleplayer with no internet. To play on a multiplayer server you do need to have internet.
Yes, there is multiplayer in Minecraft.
Download the latest version from internet.
You can play multiplayer as long as you have internet. Internet cable or not.
When you download the game after buying it, you have to sign in in order to play on multiplayer servers. However, you can still play the game without signing in to a valid account and play single player.
Any modem can be used... if you want to play multiplayer, you should use a modem which can connect to the Internet.
no you need minecraft beta to play beta servers
In a sense, yes, but in another sense, no. You do not need to install Minecraft to play it on a computer, you can simply run it in your internet browser on Minecraft.net. However, if you want to run Minecraft without an internet connection, or without visiting Minecraft.net, you must install Minecraft on your computer. You can install Minecraft on multiple computers under the same account, but your saved worlds will not sync between computers.
Minecraft doesn't need an internet browser. It's a a program that you download and play directly from your computer. However, the free version can be played on an internet browser, and you can play it with Internet Explorer 9. But I strongly recommend you get the full version, because the free version is nothing like the real game.
No.