Using headphones with desktop speakers

I just got into using Skype, a program that allows you to verbally talk to others, and I bought a headphone (with microphone) to use with the program. What I'm looking to accomplish is to have it so that the voice of whomever I am talking with comes through to the headphone, and at the same time all other sounds come through to the desktop speakers. I took a look at my sound card and noticed five holes: yellow, blue, red, green, and black. The headphone has two prongs, one red (obviously the microphone), and one black (which I figured were for the ear pieces). So I plugged the two prongs into their corresponding holes in the sound card, and left my desktop speakers plugged into the green hole. I figured it would work this way, but all the sound (including the voice of whomever I was talking with) came through to the desktop speakers. Is there a way to get it to work the way I want it to? Also, if someone could brief me on what each of the five holes on a sound card are for, I'd highly appreciate it. I know that the red is for the microphone and green was for the desktop speakers, but then what are yellow, blue, and black for?