Taps are decorative blocks which also have some functionality to them, they are mainly used outside. They were added in the Garden Update (along side Hedges, the Bird Bath, the Stone Path and White Fences).
When Right clicked on, the message "Place 3x3 water block sources under the layer the tap is on, with the tap in the middle" will appear in the chat log. To make the Tap function you need to follow the message. After you have followed the instructions it will look like the picture besides. You can then fill in above the water and under the tap with blocks to cover the water.
After you have set up the tap you can right click it and a water source block will appear in front of it. This will give you the ability to have a infinite water source for you to pick up with a bucket without the infinite water being seen .
If you break the tap while a water source is present, it will spread water like a normal source block. To remove a water source, place an object over it and remove it.
The Tap can be made using the image below as a guide. It requires 4 stone and 1 iron ingot. Place 1 iron ingot in the middle top, 3 stone across the middle, and one stone on the bottom right.