The term “family crest” is a misnomer. The first crests were assigned to individual knights, who needed to be able to identify themselves to their followers. Later, the knowledge of the meanings behind the colors and symbols on each crest became known as the science of heraldry. Many people use the term “family crest” and the term “heraldry” as if they mean exactly the same thing. A family crest might also be called a “coat of arms”.
An official coat of arms is supposed to have some specific things. At the top, there should be a motto. Below the motto is the crest, which sits on top of the shield. There will be a shield, which will have important colors, placement of symbols, and a certain amount of divisions that is particular to one family name. If the coat of arms has any supporters, they will appear on either side of the shield. Not all coats of arms will have supporters. Supporters are usually two animals, birds, or people.
In addition to the required, official, items, some coats of arms will contain more. The banner that the motto sits inside can be whatever the artist wants it to look like. Some will have a wreath that appears under the crest, and above the shield. There may or may not be a helmet in the middle of the wreath. The actual shape of the shield has changed over the centuries, and now can appear in whatever shape the artist likes. There may be a mantle added to the supporters, in between the supporters and the shield. Some family crests will add the surname of the family underneath the design. The font used for the name, as well as the design of the banner it sits on (if any), is also left for the artist to decide.
Your family tree might include a copy of your family crest. Not all family trees will, and it is up to your own, personal, tastes if you want the family tree that you are creating to include a family crest. I don’t think all families have their own coat of arms, so you might not be able to include one.
All of the symbols, colors, and divisions of the shield have a specific meaning. The art of heraldry is the study of what all these things mean. Much can be written about this topic, and there are several websites that can be used as resources if you are trying to find out more about your family crest.
Image by The Laird of Oldham on Flickr