The word “Tzaddik” is mentioned quite frequently in Jewish liturgy and scripture, and yet it is also a term used lovingly for children who do good deeds. There is a joke in my neighborhood that you can go the local store in the afternoon before Shabbos and see many tzaddikim — the little children who take time out from their games to run to the store and get their mother something while she is preparing for Shabbat! (i.e. because so many of our requests to our children at this time begin with “Joseph, be a tzaddik and get Ima some margarine… or Yael, be a tzaddekes (female tzaddik) and get Ima some more tomatoes!”
A tzaddik is translated as a “righteous person.” This is an imperfect translation, but it works, for most purposes. Much of the time, in the Torah, a tzaddik is simply a very good person, and a rasha is a wicked person. However, there are many gradations between these two extremes, which is one of the reasons that Rabbi Schneur Zalman of Liadi, in his seminal work on Chassidic thought, The Tanya, identified a third category: The Beinoni, or the intermediate type of person (however, this person is far from average, but this is another topic).
According to Jewish mysticism, a tzaddik is not just someone who has an excellent moral character, but a person who is without sin and who has a special connection to G-d. This is the way a “tzaddik” is understood by most people today. It is said that there are only 36 people who fit this description in any given generation, and one of them is the potential Messiah. Although many people associate the term “tzaddik” with famous Torah sages, such as the Baba Sali, the Chofetz Chaim, and the Lubavitcher Rebbe, many tzaddikim are not well known, and may even be obscure. There are many stories in Jewish lore about “hidden tzaddikim” who would do good deeds in private and hide their great wisdom. As soon as it was discovered that they were tzadikim, they would run away in the middle of the night and move to a new village where no one would recognize them.
Many people go to tzaddikim to receive berachas (blessings) or advice. For tzaddikim who have left the world, it is possible to go to the gravesite and pray there for one’s needs. Some people have the tradition of putting a letter into a book of the Lubavitcher Rebbe’s letters and receiving a “response.” Although not everyone agrees with this practice, there is a traditional basis for it, and many people receive guidance and inspiration this way, as if they are writing to the Rebbe.