The purpose of the rabbi at a wedding is to ensure that the extremely detailed wedding halachot, as set out by the Shulchan Aruch, are followed correctly. If not, the wedding might not be kosher.
 
A Jewish marriage includes: a written document (ketuba); a financial transaction (ring) in front of two kosher witnesses; and physical intimacy. 
 
Witnesses
 

• The witnesses must be shomer Shabbat Jewish men.
• The bride and groom should confirm verbally that these are the two (and the only two) witnesses that they want.  

 
Ring

• The groom must confirm that he acquired and owns the ring.
• The groom must say Harei at mekudeshet li ...  to the bride and place the ring on her (index) finger.

 
Ketuba

• The ketuba must be kosher.
• Before the chuppa, someone (anyone, including women) must fill in (no safrut is required for this):  the Hebrew names of the bride and groom; the date the wedding is taking place; the wedding location (city, etc.); and that a kinyan was made.
• The ketuba must be signed by two kosher witnesses.

Mezuzot are not required by halacha to have covers (sheitels). Covers are only for decorating and protecting the parchment; their use is recommended in the Shulchan Aruch.

The limits of kilayim keep:

• Individual creations true to themselves, in the way they were created by God, and
• Different, or opposing, spiritual forces governing creation in their own places and within their own bounds.

In the holiness of the Tabernacle or Temple, where opposites were peaceably and constructively resolved, kilayim in the form of sha'atnez was not only permitted but formed the foundation of all main tapestries and two of the High Priest's garments. Kilayim-type laws apply today to:

• Animals (not yoking an ox and donkey together; not interbreeding, say, a horse and a donkey to produce a mule),
• Food (not eating milk with meat),
• Clothing - not wearing a garment made of a mixture of linen and wool (sha'atnez), and
• Plants (interplanting, interbreeding, and grafting different species).
  Note Vineyards in Eretz Yisrael may only be near fruit trees if:
  1. A wall divides fruit tree and vineyard, OR
  2. The vineyard is not a bona fide vineyard. A bona fide vineyard has at least 5 grapevines in at least two rows, with at least two vines in one row and three in the other. 

Cheese/Gvinas Akum
 
Gvinas akum is cheese which has been made by non-Jews and by rabbinical prohibition is only kosher if a Jew was present during the cheese making OR if a Jew put the rennet into the milk.
Note If a Jew owns the milk before processing, a non-Jew can add kosher rennet as long as it can be confirmed that the rennet is kosher, even if no Jew is present during the cheese making.
 
Origin of the Problem: Chazal were concerned that the rennet used to make cheese might be from a non-kosher animal or even from a kosher animal that had not been slaughtered properly. Shulchan aruch says that even cheese curdled by kosher plant enzymes (such as fig branch sap or substances from certain thistle plants) are subject to the takana.
Note Gvinas Akum is not related to chalav yisrael; they are separate halachot.

Note Even rennet-less cheeses need hashgacha (religious supervision), but some non-hard cheeses may be an exception. Ask a rabbi.

“Be holy because I, the Lord your God, am holy.” Leviticus/Vayikra 19:2
 
Halacha ("The Way To Go" or "Way to Walk") guides proper Jewish behavior in all aspects of life, each day of our lives--not just in civil laws or court situations. Halacha teaches us how to behave with our families, relatives, and strangers as well as how to fulfill our religious requirements between ourselves and God.

To fulfill our role as a holy people, we imitate God's actions. Examples are visiting the sick, welcoming guests, giving charity, refraining from creative activity on Shabbat, and promoting peace between husband and wife (shalom bayit).

The true reason for following halacha is because God commanded us to do so.  We observe halacha to please our Creator and to become spiritually close to Him by doing His will and imitating His actions.

Like the word for the whole body of Jewish "laws," each rule of how to act is called a halacha (plural, halachot).

Where Do Halachot Come From?

Although you will find halachot on this site that were born only a few days or a few decades ago, the body of halacha has been around since before creation.  "God looked into the Torah and created the world," says the Zohar, and so we find the Patriarchs followed halacha even before that great law book, the Torah, was given on Mount Sinai four centuries later.
 
Many halachot are specified in the Written Torah (Jewish Bible). These halachot correspond to fuller and more detailed halachot given orally (Oral Torah) to Moses on Mount Sinai to explain the Written Torah that he received at the same time. Many halachot could not be understood from the Written Torah without the Oral Law (for example, what should be written on a mezuza scroll?) and many common practices such as making kiddush or what tefilin should look like are to be found nowhere in the Written Torah.
 
Since the Torah applies to all generations, the Torah specifies that there be wise and learned people to decide how to apply halacha to the situations of the day.  Halachot can be found in sourcebooks such as the Mishna, Gemara, their commentaries, Shulchan Aruch, Mishna Berura, and responsa (questions and answers originally sent by letter and now, occasionally, by email or SMS!) of later rabbis.
 
Sometimes a custom becomes a halacha, sometimes not.  For example, the original halacha for tzitzit was that a Jewish man who wears a four-cornered garment must have tzitziyot on each corner.  The custom, which has become universally accepted and now has the force of halacha, is that Jewish men wear a four-cornered garment in order to be able to fulfill the commandment of wearing tzitziyot.  An example of a custom that did not become a halacha is that some men and boys wear their tzitziyot outside of their shirts and pants.

Levels of Halachot

In halacha, there are three levels of what to follow or observe. They are differentiated on this website by the following terms: 

• “Must”:  Halachot that are generally non-negotiable except in extreme situations;
• “Should”:  Customs that have been accepted by the entire Jewish world (or major segments of it) and that may be overridden when necessary, sometimes even if not extreme circumstances; and
• “Non-Binding Custom”:  Customs that are not universally followed and that do not need to be followed except by people who have the tradition to do so.