You can modify the controls from the Options->Controls menu. Below is the default keyboard and mouse controls:

W   Move Forward
S   Move Backwards
A   Sidestep Left
D   Sidestep Right

Q   Drop Item
E   Open Inventory
T   Chat with other players
/   Enter a console command

1-9   Selects item on the Hotbar (See HUD Section)

SPACE   Jump
TAB   List players on the server
LEFT SHIFT   Sneak (Stops you falling off the sides of blocks)

MOUSE BUTTON 1   Attack (Hold down to destroy Blocks)
MOUSE BUTTON 2   Activate object or Place object
MOUSE BUTTON 3   Pick Block (Creative Mode Only)

Your health is shown by the Hearts (Number 1) on your hud. When you are poisoned the hearts turn green. How hungry you are is shown by the food icons (Number 2), the more food icons there are the more full (and less hungry) you are. When you have 8 or more food icons your character will regenerate health over time. If the hunger reaches 0 food icons your character will loose health over time, eventualy dieing. You can decrease your hunger by eating food, cooked food gives you more food icons than un-cooked food.

The experience bar (Number 3) shows how much experience you have as well as how much more you need to get to the next experience point. As you gain experience (By mining and killing mobs) the bar will fill up with green, once its full your experience points will go up and the bar will become empty. Experience points are used in Enchantments. The Hotbar (Number 4) shows the items/blocks which you can select and which is currently selected(And usable).

Your inventory allows you to do basic crafting using the 2x2 crafting space (Number 1). You can equip armor using the top left slots (Number 2). Number 3 shows the main storage space and Number 4 shows the storage space for the Hotbar. In total you can hold 36 stacks of items.

You can move items by left clicking on the item you wish to move and then left clicking on the place you want to put it. If you hold Left Shift while you click on an item you will move it between your main Inventory Space (Number 3) and your Hotbars Space (Number 4). If your looking inside a chest holding Left Shift when you click on an item will move items between your inventory and the chests. If you Right Click on an item stack in your inventory you will pick up half of the stack. If you Right Click when your holding items you will drop 1 item into a slot.