devnp,
Checkout the following:
Wikipedia: http://en.wikipedia.org/wiki/I%C2%B2C
I2C-bus.org Primer: http://www.i2c-bus.org/i2c-primer/
I2C spec: http://www.nxp.com/acrobat_download2/literature/9398/39340011.pdf
Note I2C is sometimes called "IIC" as the full name is Inter-Integrated Circuit bus.
If you Google search for I2C you will find lots of other links as well.
IC is usually just short for "Integrated Circuit". I have no idea what I3C is. When you google search I2C and I3C, there is a "i3C" which appears to be a brand name of an I2C core for ASICs/FPGAs.
Hope this helps,
Cheers,
Alan