I'm downloading Minecraft for the first time. Will I need to buy anything?
You're not actually downloading Minecraft — you're downloading the official game launcher.
That launcher will, given valid log-in credentials (obtained by signing up and purchasing the game), download Minecraft and allow you to play it. So by not paying, you're missing out on the entire game, as it were.
You can play a free version of Minecraft here so you can get the basic idea of the game. But beware, the current version offers so much more than this classic version that to really make up your mind you should have played the full version.
To name few things you are missing out:
- Crafting (huge part of the game)
- Redstone circuits
- Survival mode (fight against mobs)
- Enchanting
- Alchemy
There are certainly many more I forgot to mention.