Loyal readers of this blog know already how I feel about challah. On this page you will find a bit of background on it and some basic Jewish law (halacha) on the topic.
At the bottom of this page, you'll find all of the Challah related recipes and braiding techniques.

The Torah source for the commandment of separating challah can be found in the book of Numbers. It is one of the twenty four gifts meant to be given to the high priests as they themselves had no means of independent financial support. In the land of Israel this mitzvah is a Biblical, as opposed to Rabbinic, obligation. Furthermore, there’s a beautiful verse in the book of Ezekiel that says that he who does separate and gives challah  to the high priests brings blessing into his home. The importance of this mitzvah is compounded by our sages' statement that this mitzvah is one of the few for which this world was created.

As many may know this commandment of challah has special significance for women, not because it involves baking, but because it completes the trio of commandments specifically given to women, the other two being candles and family purity. Furthermore, our sages tell us that challah is meant to atone for the sin of Adam and Eve in the garden of Eden which happened on a Friday. Therefore there is added significance to performing this mitzvah on Friday afternoon.

The time of kneading the dough and the separation itself are known to be special times for prayer. One may pray for herself or for others who may need a speedy recover, to find their match, or to have children. There is a known segulah for women in their ninth month of pregnancy to take challah to ensure a safe and easy delivery.

Basic Jewish Law
What requires separation?

Any amount of dough from which challah may be taken that is made up of the five grains (wheat, barley, spelt, rye, oat) requires separation.  This includes a mixture of any of the grains. (Oat flakes are included, regardless of the fact that they haven’t been ground into flour). One may only take challah with a blessing if the dough is made up of the following liquids: water, wine/grape juice, olive oil, honey, or milk. 
The dough must be intended to be baked in order to be blessed. A dough that is meant to be boiled or fried does not get a blessing. It is preferable to bake a small piece and then proceed as you would with the rest in order to be able to bless it. A dough that is boiled first and then baked such as bagels receives a full blessing.

There are a lot of discussions as to how much is required to be able to separate and how much is required for a blessing. Ask your personal Rabbi. 

Here are some general guidelines:
-Less than a kilo or two pounds does not require separation at all.
-1.2 kilo- 1.67 kilo- separate without a blessing
-1.67 kilo- 2.25- some separate with a blessing, some do not
-More than 2.25 kilo or 5 pounds requires separation with a blessing

Who performs the separation of challah?