Opening with Knight to f3

I was playing against the CPU and they opened with Knight to f3. What is this called & what is the best defense against it

Reti Opening or Barcza System.

There are lots of decent defenses, but none that is specifically "best".

As Black, you can play it like a Queen's Gambit Declined, or like a Caro-Kann, or like a French Defense, or like a King's Indian Defense...

As White I wouldn't play it like that, though.

1. Nf3 c6 and now either 2. g3 or 2. c4 d5 3. b3.

How does c6 force white to play d4 or c4?

The point of Nf3 is that it's very flexible, and white can transpose to many openings depending on black's reply. From experience, I find white usually plays d4 at some point, so I usually respond with 1... d5 or Nf6 depending on my mood. 

Thanks very helpful & no I need to learn about Slav.

Nonsense.

One move does not an opening make.

It will probably transpose into something else, unless White keeps true to the Reti and Black cooperates.