Bab Talmud, Moed Kattan, 27b
Formerly, the expense of the burial was harder to bear by the family than the death itself, so that sometimes they fled to escape the expense. This was so until Rabban Gamliel insisted that he be buried in a plain linen shroud instead of costly garments. And since then we follow the principle of burial in a simple manner.
A kittel is a white garment, normally made of linen or cotton. It resembles a dressing gown, with a belt of the same material. Kittels are normally quite long and thin, with little decoration.
A kittel is first and foremost a burial shroud for Jewish men. Jewish burial practice demands a simple funeral to prevent rich people from being overly extravagent and shaming the poor.
Although death is the last time anyone wears a kittel, it is not necessarily the first. There are a number of times when Jewish tradition calls for the use of a kittel:
Observant Jews fast on the day of their wedding. This is because a wedding is seen as a new life, like being reborn. In particularly obsevant communitites, the groom wears a kittel over his clothes on the Chupah, during the ceremony. This is to remind him of his mortality, and to repent. It also represents purity. One custom is for the best man to dress the groom in the kittel
During the Seder, the most important family ceremony in the Jewish calendar, the officiant -- that is, the bloke running the Seder -- often wears a kittel. Some Rabbis suggest that it is an adaptation of the High Priest's linen garment, and so represents the Seder nights in Jerusalem during the time of the Temple. Also, the reference to death is supposed to be a check against too much merrymaking (important on a night where the service involves drinking four cups of wine).
During the prayers on Yom Kippur, (which is pretty much the whole day) it is customary to wear white. Once again, men often wear their kittels. Here, they symbolise purity; by Yom Kippur, Jews are supposed to be freed from all sins, like angels. Again, they are also supposed to remind those praying that their lives hang in the balance, and so to repent properly. In some congragations, people also wear Kittels on Rosh Hashanah for similar reasons.
For many observent Jews, then, a kittel is something which you get (or are given) at your wedding, and wear only once or twice a year until you are eventually buried in it. There is a certain neatness of purpose there which is hard to get around; it is literally a garment for life -- and death.