THE RED HEIFER
The red heifer is a cow that is reddish-brown in color. The People of Israel (i.e. the Jewish People) were commanded to use the ashes of a red heifer in the purification process of those who have contracted ritual impurity as a result of coming in contact with a human corpse.
The burning of the red heifer is one of the 613 Jewish commandments. In the words of Maimonides,  “He commanded us to prepare the red heifer, so that its ashes be ready for one who needs it for purification from the impurity of a corpse."
The complete renewing of all aspects of the Holy Temple service, and the revival of complete ritual purity among Jews, is contingent upon the preparation of the red heifer. The performance of this commandment is incumbent upon the Sanhedrin and the kohanim (priests), since its performance entails details that must be under public supervision, such as; the purchasing of the heifer, the process of its preparation, taking caution against possible impurities, the meticulousness required with the slaughtering and burning process, etc.
In the past, this process was performed, on average, once every fifty years, and every generation must prepare itself to perform this commandment, in all its details.
In our generation, in which we look forward to the reinstitution of the service in the Holy Temple, with G-d's help, the Temple Institute has conducted research on this topic and the teachings have been published in articles and books. A condensed review of this material is presented here.
THE IMPORTANCE OF THIS COMMANDMENT AS A PREREQUISITE
TO FULFILLING THE COMPLETE SERVICE IN THE HOLY TEMPLE
The preparation of the red heifer is a precondition for the reinstitution of the complete service in the Holy Temple.
Although in a situation when there are no ashes of the Red Heifer, those Temple services which are designated to be performed on a specified day, must be conducted even in a state of ritual impurity, such as: The Tamid – Daily Offering, the Musafim - Additional Offerings for Sabbath and festivals, the Paschal lamb, etc. However, all the other offerings which do not have a designated time-frame may only be brought when the officiating kohanim (priests) and the individuals bringing the offerings are ritually pure.
Furthermore, entrance to the Sanctified Courtyard for the performance of services that are not time-dependent, is only permitted to those who are ritually pure. 
In addition, there are commandments that are to be performed outside of the Temple compound only by a person in a state of purity. For example, the commandment to separate a tithe from produce (terumah), and separating part of one’s bread (challah), for these may only be given to the kohanim (priests) when the item that was separated and the kohen are ritually pure. Likewise, the practice of Second Tithes (ma'aser sheini), which are to be separated from the produce on the first, second, fourth and fifth years of the seven-year cycle and are to be eaten by the People of Israel within the walls of the sanctified city of Jerusalem, may only be performed in a state of ritual purity.
THE RED HEIFER - ITS COLOR AND PHYSICAL PERFECTIONS
The heifer must be completely red, meaning, there must be no other colored hairs among the red ones.  It appears to be that this “red” color referred to is actually a variety of different shades of red that have been found in cow hair throughout the generations. For example, a brownish-red hue that is more red than brown, as well as orange, pink, burgundy, and the like.  The Jewish law pertaining to the red heifer hair color requirement is as follows: If the cow has two white or black hairs next to each other – it is invalid.  Even if those hairs are plucked – the heifer is still invalid.  The eyes of the heifer, teeth, and tongue do not have to be red. However, if the horns and hooves are black, they must be removed without harming the flesh of the heifer, in order for the red heifer to be valid for this commandment. 
THE DISQUALIFICATIONS THAT PREVENT THE USAGE OF THE RED HEIFER
As stated, the language used in the Torah regarding the red heifer is “perfect.” And, according to the simple text of the Torah, “perfect” is to be understood in its broadest sense, including a heifer “on which no yoke has been laid.” From here the Sages learn that if one were to perform any labor with the red heifer, even if someone were simply riding it or leaning on it, it becomes invalid. Even if a yoke was simply placed on the red heifer and it was not yet used for labor, it becomes invalid.  Furthermore, it also becomes invalid in the event that a bull attempted to mate with her. 
However, disqualification does not occur if the work with the heifer was performed automatically and against the will or intent of the owner. Additionally, if the work is performed with the red heifer in order to maintain the heifer’s needs, such as tying a rope around it for its protection or leaning on it while washing it, etc. it does not become invalid. 
The Torah also states that the red heifer must be “without blemish, in which there is no defect”, which is to be understood that the heifer cannot have any physical defects or blemishes that would invalidate any other offering.  So, too, anything that generally disqualifies offerings also disqualifies the red heifer to be used. For example, if it falls under the halakhic concept of treifah (literally “nonkosher,” torn, mangled). This is because the red heifer is referred to in Scripture as 'chatat', with similarities in law to the laws of a Sin-Offering. 
As for the time the red heifer may be used for this commandment - the Sages state that it is valid when it reaches maturity, i.e. from the beginning of its third year.  From this age and onward the red heifer is valid without age limitations. Though, ideally, one should not wait till it passes the age of four, lest it grows hairs of colors that deem it invalid. 
PURCHASING THE RED HEIFER
The red heifer is bought with the funds of the Temple Treasury.  It is also possible to purchase one from a Gentile or from outside the Land of Israel.  The red heifer cannot be purchased when it is still a calf, in order raise it until maturity under the auspices of the Temple Treasury. Rather, the red heifer is to be purchased with the funds of the Holy Temple when it becomes of age of a matured cow.  Until then, the red heifer is raised by its original owners, under the supervision of the Jewish Court. 
THE HOLINESS OF THE RED HEIFER
The red heifer is not considered an offering (‘korban’), since it is not offered on the altar in the Holy Temple. Therefore, it is not defined as "Sanctities of the Altar", rather it is under of the category of "Sanctities of the Temple Treasury", as all items owned by the Holy Temple. It is for this reason, in the event that it died unexpectedly, it is permissible to redeem its value from the Temple Treasury in order to make use of its precious hide. However, in this case, its flesh is not given to a Non-Jew or to animals to eat (even though they do not have to abide by laws of kosher food).
Despite the above, the red heifer ritual also abides by some of the rules of offerings in the Temple since the Torah uses the same word to describe both Sin Offerings and the red heifer (‘chata’at’).