Inventory Editors are programs that put items directly on your person in Minecraft, giving the player unlimited ores, building blocks, tools, weapons, food, water, and lava as you would ever need. It also allows stacking of items normally not stack-able, such as porkchops. One other ability is the ability to put up to 240 of an item on one square. I will show you the basics and how to do these things in the tutorial.
Download the editor here:
Click the red "download" button,
Now select "Save File" and ok.
Go to your downloads folder, open the folder containing the file, then unzip it using WINrar or 7zip, the file name should look something along the lines of "2010_11_20_INVedit_v3_by_bloc97(2)"
Open the extracted folder, then launch the INVedit program.
Hit the "New" button to load up a fresh inventory.
Load up your world.
Go to the program and select "Open World X", with X being the world you'll be using. In my case World 5. Your inventory will copy itself from the game to the program.
Your inventory will now be copied, just use the sidebar to get the items you want. Now save it back to the world you'll be using. Once again, in my case World 5
Now open up your world, and you'll find the items waiting in your inventory for you.
Enjoy the loot :D