It's not free. You have to sign up on the website and buy minecraft. When you do, you'll have access to play either online or by download.
Basically, it's this extremely addictive game, where you start out in a forest, in a blocky world. You try to break trees with your hand and collect wood. Out of the wood, you can construct a table. With the table, you can construct more stuff. With the more stuff, you can construct more and more stuff.
When you look at videos, it may seem complicated at first. But actually, it's really easy to get the hang of and extremely addictive
Also a very creative game.
Not only can you build whatever you want, but it's also a game of survival if you like. (You can turn on enemies in the settings and zombies will attack you unless you attack them or like protect yourself with shelter or something)
So yeah, overall, my conclusion is:
Minecraft=Get creative, build. Build from what you build. Build from what you build.
Hope I helped
PS. The game's easy to pirate but c'mon it's 10 bucks, show some appreciation for a creative developer like that