I would say there isn't much difference between a center forward and a striker positionally. I would consider a center forward more like a big, old fashioned target man like Emile Heskey. A striker would be someone like Michael Owen, a small, often quick forward who's job is to stick the ball in the back of the net. I would consider Mario Gomez a center forward, even though he scores a lot.
A second striker would be someone like Dennis Bergkamp when he played with Henry at Arsenal. He dropped back into midfield, linked and created chances for the finisher Henry. Generally now, second strikers are being used as lone strikers, like Robin van Persie now.
However, the original article is still good for people new to the beautiful game.