This is somewhat of a catchall term for the subtle ways criminals, travelers and outcasts have been able to communicate without being detected.
Methods will vary from region to region and city to city. In Ghal Pelor everyday conversation is the mask used to conceal conversations about dangerous jobs, police activities and Black Thorn Society gatherings. It generally comes in the form of a conversation about a fake mutual acquaintance. The acquaintance's name in conversations about a job usually start with "how's your friend (insert random name", I haven't seen him/her since that time at (where the job takes place or near to it.). The general health and well-being of the fake acquaintance will generally indicate the difficulty of the job, the profitability of the job will be disguised with details about the acquaintance gaining or losing weight. These conversations can be very nebulous but they're only designed to exchange vague bits of information and and gauge interest. These conversations end with the party arranging to meet and discuss the full details at a later time. While messy at first glance Ghal Pelorian's Thieves' Cant is all about context and innuendo and the players involved quickly acclimate to each other's meanings and flow.
Some terms are relatively common in these conversations: Mom or Mother: The Black Thorn Society, Reference to taking classed at the University: Incarceration, Visiting Dad or Father: Dead. There are numerous other common terms.
A number of criminal elements in the Farmlands of Ghal Pelor and the wilderness areas around them opt for nature calls instead. These tend to convey less information and can easily be misconstrued but they have the benefit of being veiled in otherwise natural sounds that few outsiders would think to question.
In Baradrad the code language can be conveyed subtly by certain movements undead servants can be programmed to make and respond to at the will of the masters. They also communicate through non-verbal gestures made by the living as well.
Elves tend to have "House" dialects. Variants in the common elven tongue that are commonly know only within their Houses. The words often do not matter so much as they the tones. Elves are expert linguists and their language is so old and dense that they gather a great deal of meaning from tones and subtle emphasis. This is the same with their version of the Thieves' Cant.
Frostmerites version of a Thieves' Cant is a language cobbled together from the words of the other kingdoms. Because The Common Tongue is not widely spoken in the north shadowy conversations can me concealed simply with bastardized versions of foreign languages.
In addition to spoken words the Cant can come in the form of signs, symbols and marks informing those who notice them of dangers and places of safety. It's generally considered bad form to lie about information left in written form. It's also good practice for those that speak the Cant to leave marks when they can.