User Interface

Overview

UI stands for "User Interface". The user interface for Minecraft is pretty simple. Here's an overview of the components: Note These images and descriptions are based on the standard UI of the game - If you have mods and resource packs installed they may look different.  

Armor

This is a gauge or meter that indicates your armor "protection level". Basically the more armor you're wearing (And the harder material it's made from) increases your resistance to attacks and damage.
To maximize your armor level you will need a full set of diamond armor. This meter does not consider enchantments. Enchantment protections are in addition to this level meter. Each icon on this meter is 2 armor points
Learn more about armor here.

Health

Health hearts indicate how healthy you are, or in other words, how close you are to dying. If all of your hearts become empty, you will die.
Each heart has a value of 2 points. So taking a small amount of damage will deplete 1/2 a heart at minimum.
As long as your hunger meter is full, hearts will replenish (or "heal") at a slow rate. You can heal faster by drinking potions.

Hunger

You have to eat periodically. These meat icons indicate how hungry you are. Like the other meters in the UI each icon is a value of 2 and the hunger meter slowly depletes by 1/2 a meat.
The more active you are, the faster you become hungry. If you let this meter become empty, you will begin to lose hearts and starve to death. Learn more about food here.

XP & Player Level

When you kill monsters in game, breed animals, trade with villagers, and smelt minerals, you will see little green orbs appear, which are attracted to you and eventually you will "absorb" them when you get close enough.
These are "XP" orbs or "Experience Points" the more of these green orbs you collect the fuller your XP meter gets. (The green below the other meters). When your XP meter fills up, you gain 1 level. That's what the green
number is above the meter - How many levels you've earned. You can trade in XP levels for enchantments (enhancements to tools, weapons & armor) and repairs in the game. Learn more about XP & Leveling here.

Hotbar

  The hotbar is where you "equip" your most common tools, items and materials. This allows you to access them quickly. You select what you want to use by scrolling  your mouse wheel or pressing the 1-9 keys at the top of your keyboard.  

Player Inventory and Crafting Recipes

By pressing "E" you can bring up your inventory screen. The screen is divided into sides. On the right side is the inventory and on the left side is the crafting recipe book. You learn recipes as you discover new materials. (or you can look them up online. :D )

Other sections include:

Armor

This is where you wear your armor. Drag armor items from your inventory to these squares.

Left Hand

Also known as "off hand", you can hold items, tools and shields in this hand.

Simple Crafting

This is your simple crafting interface - where you can create items before you get your full workbench. Learn more about crafting here.

Player Inventory

Here is everything you're holding in your "backpack" so-to-speak.

Hotbar

See above, this is where you drag and place/remove items from your inventory to your hotbar.