3. d4 is the Scotch game. The basic idea of this move is pretty simple: a) to develop, b) get rid of blacks central e5 pawn. Notice that after 3. Bc4 or 3. Bb5 white also often plays d4 break at a later stage so this pawn break is very usual procedure of applying pressure in the centre. Blacks two main replies (after 4. Nxd4) are 4... Bc5 or 4... Nf6 as you did. I think 6. Nc3 is lightly unusual. The main continuation for white here is 6. e5 Qe7 7. Qe2 Nd5 8. c4 and now either Ba6 or Nb6.
What is this opening?
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Yup, this is the Scotch game but instead of 6.Nc3, white should play 6.e5 and you go into the ultra-sharp Mieses Variation. I love it. Black has a lead in development but white has a superior pawn structure.
Yes, your opponent definitely played badly. He threw the reputation of the Scotch out of the window.....if you want, you can join the group The Scotch fans Team. You'll learn its greatness there.... 
Are there any effective ways to counter this opening? What is the purpose and what are the main lines of attack etc...? I found the opening really interesting as i'm a regular e4 player for white and black (occasionally i try the sicilian defense but i'm not well-versed in it either).
Any help is much appreciated =]. I've gotten the basics of chess and i'm now trying to learn new openings/defenses and most importantly their purpose, main lines and threats.