The Inter-integrated Circuit (I2C) is a multi-master, multi-slave, serial computer bus invented in order to simplify the board schematics, thanks to the fact that it needs two wires only (apart from the GND) to do its job. It's widely used in embedded computers to connect on-board sensors/actuators to the main CPU.
Despite the fact that the I2C bus is a multi-master, a typical configuration is a single master device (the CPU) connected to several slave devices (the sensors/actuators); for the USB bus, the master directs all the transfers. However, a main difference should be outlined: an I2C device can have a dedicated interrupt line connected to the CPU that can be used to signal that a message must be read by the master (in the USB bus, the interrupt messages go over the bus too!). So, a simple I2C connection needs two wires while they only, in case of interrupt lines, need three or more lines.