Welcome to interface-i2c.org!
I2C is the 2-line wire interface, which is used widely in electronics devices and components. It is created by Philips.
This website contains the theoretical information on I2C interface, practical designs and open source examples, which use microcontrollers with support of I2C interface. You can view schematics and electrical diagrams or select them as reference to use in your projects.
There are plenty of examples available and the list is constantly enlarged.