Nerd?

After work, I went home to the middle with my co-worker. It was the last in the beginning that I worked together with him. It was heavy rainy. Although we were walking under umbrellas, we were all wet with rain.

Before I started using computers, I used to be afraid of being nerd, so I asked him, "Am I a nerd?" He said, "No. You are not a nerd, you are a professional."
What is the difference, nerd and professional?