An iButton is a chip housed in a stainless steel enclosure. To keep the cost of access low, the electrical interface is reduced to an absolute minimum, i.e., one data line plus ground (also called 1-Wire®). The energy needed for communication is "stolen" from the data line ("parasitic power").
In some ways the family of iButton devices vary greatly. There are iButtons that contain only a unique registration (ROM) number and there are iButtons that have read/write memory, real-time clocks, and temperature loggers. However, with respect to mechanical and network level commands all compliant readers can read them.
This document explains all of the relevant specifications for mechanical, electrical, and protocol for the iButton family of products. Most of the information is also applicable to the 1-Wire products not in the iButton package. Read full article.