First and foremost you need to understand that there are two stages of detention: prison and the area it is, accordingly, the detention and the place of punishment.

A letter in jail

The internal regulations of the detention facility inmates are allowed to send and receive letters in unlimited number, they are sending and receiving is carried out by prisoners through the administration.
Of course, the correspondence is censored, that is, a special person (the censor) reads the letter and makes a decision, the prisoner will receive a letter or not. Therefore, try to write letters more narrow-minded character, do not write about any details of a court case, especially about some of the activities that fall under articles of the criminal code, as it can be used against the recipient.
Remember that through the people who read the letter, the information can get to the investigators, prosecutors, and hence it may affect sentencing, or simply to stop the prisoner. Do not write information relating to the communications prohibited in the prison (e.g. mobile numbers).
The letters don't put photos or drawings frivolous nature, the most important is the observance of the internal rules of the prison and the rules of the penal code (erotica, for example, in places of deprivation of liberty is prohibited).
Remember, if you put something in an envelope, to make sure that the recipient is really going to get to do inventory. By the way, is not superfluous to attach more clean envelopes and stamps, as in prison they are almost at their weight in gold.
A letter to the area

In this case, any significant differences from the procedure of correspondence with the prisoner in the jail there, although you can pay attention to the following points.

The censor of the area could be not a COP, but one of the prisoners. This is not a rule of conduct, and the situation in which the chief among the prisoners, "looking" solves most of the issues of domestic life. There, where the power is, as expected, is in the hands of the staff, the rules are as strict as in jail. From the response letter you will understand in what type of facility were your family member or friend: letters of the zones of the first type differ in the relative freedom of presentation, you can meet the attacks on the leadership of the colony, description of living conditions. Other letters are written almost "a blueprint" and only that I miss that was corrected, realized, etc.