So, the Tango "Códigos" or codes (of conduct) are the basic etiquette of Argentine tango. The códigos themselves are all about respecting your dance partners, the other dancers, and the organizers at an Argentine tango event.
Tango culture developed códigos to make the dance experience safe and enjoyable -- safe from those who might injure others on the dance floor, from those who demand dances or pester others. Knowing and using the Códigos means that you understand that dancing tango is a community activity and you choose not to spoil the experience for yourself or other dancers. Adhering to them means more fun dancing for everyone.
Mark Word has written a great article on the Argentine Tango Códigos:
Mark Word organizes the Códigos into groups. So while the full article is excellent, direct, and very worth reading, here is the outline of the sections.
Chapter One: Preparation for the milonga
What to Wear
Chapter Two: Arriving at the Milonga
The Alpha and Omega Rule
Chapter Three: On the Dance Floor
Lady Leads the way?
New meanings for words you thought you knew, like "thank you" and others
Chapter Three, Part II: On the Dance Floor
Wise teachers are silent at a milonga
Wise students are silent at a milonga
Chapter Four: Near the Tables
Floor Craft Basics ...that even experienced dancers may not have learned
The different types of people
Chapter Five: To and from the dance floor
Entering the dance floor
Escorting the woman back off the dance floor
Chapter Six: After the Milonga
Going for coffee ("un cafecito")
Taking care of your aching feet!