What is the meaning of the different Audi A6 engine designations?

I'm looking at buying a used Audi A6 and I'm confused by the engine designations. Can someone explain what they mean and which ones are better?

The engine designations in Audi A6 models refer to the size and type of engine. The most common designations are TFSI for petrol engines and TDI for diesel engines. TFSI engines provide a higher performance and can be more efficient, while TDI engines offer more torque and can be better for long distance driving.