Spawning items allows you to obtain items without mining/harvesting them. Every Item and Block has its own unique Item ID.

To spawn an item you must first know the Item ID that is assigned to the item. You can find a list of the Item ID's on the ID List Page. Certain Items/Blocks have a Secondary ID which is represented in Minecraft Info's ID List as Primary:Secondary.

You can spawn items in a singleplayer game if you enabled the "Enable Cheats" option when you created your world. In Multiplayer you can only spawn items if you have the approperate permissions on the server.

To spawn an item open up the chat and type in:
/give (Username) (Primary ID) (Amount) (Seconday ID)

For example if you would like to give Notch 64 Sapling (Pine) then it would look like:
/give Notch 6 64 1

With version 1.7 and onwards you can use ID Names instead of the Primary ID Number. For example:
/give Notch sapling 64 1

Only the Username and Primary ID fields are required by the give command. If Amount is not set it will default to 1, if Secondary ID is not set it will default to 0. With Singleplayer you can also use an Inventory Editor such as INVedit, allowing you to spawn items in worlds which never had "Enable Cheats" set to true.