Minecraft's user interface provides a sleek first-person view of your surroundings. The W, A, S, and D keys allow you to move forward, strafe left, move back, and strafe right, respectively, and the spacebar allows you to jump. The left mouse button is used to attack a block of terrain or mob (the term for monsters, animals, etc.) with your currently selected tool or item, and the right mouse button unleashes any special power your tool has on any target in your crosshairs.
The health and hunger bars show the general state of your player. Don't let either of these drop. If you run out of hearts, your player dies and has to respawn without any of your precious equipment. If your hunger bar drops, then you might be unable to sprint. You could also start losing health.
Finally, the Item Hotkeys, mapped to your number keys, allow you to quickly switch between various tools and items.