TWI stands for Two Wire Interface and, for the most part, this bus is identical to I²C. The name TWI was introduced by Atmel and other companies to avoid conflicts with trademark issues related to I²C. A description of the capabilities of TWI interfaces can be found in the data sheets of corresponding devices. Expect TWI devices to be compatible with I²C devices except for some particularities like general broadcast or 10 bit addressing.
For the time being there is also no TWI high speed mode.
If you have an I²C analyser it will work with a TWI set-up as well without modifications.