Answer:
   
   The Torah teaches us, "Do not draw close to a woman when she is a
   niddah; relations are forbidden [at that time]." (Leviticus 17:18).
   From this, the laws relating to ritual purity and niddah are derived.
   
   First, note that the extent to which these laws are followed depend on
   the movement. These laws are followed in the more traditional
   movements, and are often reinterpreted in the progressive movements
   such as Reform as a mechanism of rediscovering female spirituality.
   
   So, what is "niddah". Simply put, a woman becomes a niddah when blood
   comes from her womb. She might see the flow, or she might see a stain
   on her clothing. The blood must come from her womb. If she cuts her
   finger, she does not become a niddah. If she finds blood on her
   underclothing, and she does not know if it is from a cut or from her
   womb, she needs to check with her Rabbi.
   
   Being niddah should not be looked at as a time of negativity. The
   traditional perspective is that this is when a woman's body is
   renewing itself, getting ready to produce fresh ova so that she will
   be able to fulfill the commandment of having children.
   
   A woman is a niddah until she undergoes "taharah." The taharah process
   involves a minimum of twelve days, most often thirteen. These are
   divided into two sets of time, the first five days, and seven days of
   taharah, after which she must immerse properly in a mikvah (ritual
   bath).
   
   A woman who does not go through the taharah process cannot become
   tahor (the opposite state from niddah). It does not matter if she not
   seen blood in ten years. No matter how long ago she last had her
   period, if she has not immersed properly in the mikvah, she is still a
   niddah.
   
   First, however, to the other aspect of being niddah. Recall the Torah
   verse above. It says "Do not draw close." Traditionally, this is tied
   in with marital relations, and the view is that any act that could
   lead a person to marital relations is forbidden. A husband and wife
   are very accustomed to being physically intimate with each other, and
   therefore they must take great care during the time the woman is
   niddah.
   
   While a woman is a niddah, she and her husband must relate completely
   on a non-physical level. Traditionally, they do not hug or kiss each
   other (or do more).
   
   How is the niddah period, and the various time periods determined? The
   first five days begin when a woman first sees her flow. She counts
   from the beginning of the flow, and continues until the flow stops. If
   it takes less than five days for her flow to stop, she still has to
   wait until five days are over. Even if she saw blood for only one day,
   she must wait five days until she can begin the seven-day taharah
   process. The five days need not be complete five days. The first day
   might start in the middle of the day, if she first saw her flow in the
   afternoon. But whenever they began, they end on the night after the
   fifth day.
   
   If she sees blood for more than five days, the "five" days end when
   she has definitely stopped seeing. Once she has stopped seeing blood,
   she can begin the count of the Seven White Days. "stopped seeing
   blood" means that she has stopped seeing either a flow of blood or
   stains on her clothing
   
   completely. These days begin when the woman, before sunset, takes a
   shower or bath, and cleans herself thoroughly, everywhere. She then
   waits a few minutes, and inserts a cloth and checks herself. If it
   comes out clean, then the next day is the first day of the Seven White
   Days. During this period, the woman must check herself twice a day:
   when she gets up, and just before sunset. Checking is done with a
   white, absolutely clean piece of cloth. Often, such cloths are
   available at the local mikvah. The woman first checks the cloth very
   carefully to make sure it is clean of any marks. She then places her
   finger in the center of the cloth, and allows the cloth to wrap around
   her fingers, and the pushes the cloth so that every surface inside her
   is touched by the cloth. She removes the cloth, and checks it very
   carefully. If it comes out free of any mark, no problem. If the cloth
   has a mark, she looks at the color. A red or black mark means there is
   still discharge of blood. White or pale yellow is not a problem.
   Colors like brown, dark yellow, gold, and pink, are very problematic.
   Traditional women would then bring the cloth to a competent local
   Orthodox Rabbi who looks at the cloth and is able to determine whether
   it is Niddah blood or not. Orthodox rabbis have special training that
   allows them to make this determination.
   
   Traditionally, during these days, the woman should wear white
   underwear and uses white bed linens. Of course, any staining during
   this period must be considered, as above.
   
   When the Seven White Days are over, that night, the woman goes to the
   mikvah. This is the same day of the week the Seven White Days began.
   To prepare for the mikvah, after checking, the woman takes a bath,
   followed by a shower, and other careful preparations. She cleans and
   cuts all her nails, both finger and toe, as well as making sure there
   is no food between her teeth. She cleans her ears, and every body
   cavity. She removes all makeup, and combs her hair completely. Many
   women take the bath at home, and do the follow-up shower at the
   mikvah.
   
   When going to the mikvah, she may not have anything between her and
   the water at any part of her body. Therefore she must remove all
   jewelry, makeup, etc. There is usually a woman attendant at the mikvah
   to help the woman check that she is ready for the mikvah. During the
   immersion, the woman makes sure that she is completely immersed
   (including all hair). There are appropriate blessings said.
   
   The woman then returns home, and informs her husband that she is now
   in the tahora state. Marital relations are then permitted (in fact,
   tradition dictates they occur that night). Biologically speaking, the
   best night to conceive is usually mikvah night.
   
   Note: The Torah also forbids relations on the day that a woman expects
   her period, called her "veset". She knows when to expect her period by
   keeping a careful record. Usually, a period of 30 or 31 days since the
   first sighting of blood is used.
   
   A vital factor of the Laws of Family Relations "tznius", or proper
   attitude. Jews do not make jokes about private bedroom matters. A
   woman's personal matters are nobody's business but hers, her husband's
   when he needs to know, her doctor's, and her Rabbi's when and if the
   rabbi needs to know. Women do not discuss these matters with others.
   Some specific aspects of this are discussed at
   [5]http://www.milknhoney.co.il/holy/19.html
   
   Why does Judaism have niddah? These laws are Laws of Holiness, and
   serve to elevate the physical to the highest spiritual level. It takes
   a phyiscal aspect and adds holiness to it, allowing us to use the
   physical for spiritual gain. As society has rediscovered the
   importance of spirituality, these laws are being rediscovered, and are
   even being adopted, to varying extents, by the progressive movements
   in Judaism.
   
   In Judaism, marital relations are a gift from G-d. They are neither
   shunned nor avoided. However, they are not debased either. Rather,
   Judaism provides a way to use sex to elevate us. By following Torah
   laws, we develop the self-control and discipline that can lead us to
   holiness. During the time that a man and woman are forbidden to have
   relations they are forced to relate to each other in non-physical
   ways. They must see each other in other terms, and develop their
   relationship with each other on a spiritual and emotional footing. Is
   this the reason G-d gave us this law. We cannot know. However,
   understanding this effect often provides additional understanding for
   following the law.