In addition to  [NAME]  good answer ... Like him, I mostly use "USB based" headsets and mics now-days. An old headset with "4 contact bands" (like from a Android phone or old iPhone) would work in that front jack. Here is a little "USB sound-card" that would work. You can plug your mono-channel analog mic into that pink jack. These are handy for many different uses. If you really want to use your old analog mic/headset with the Internal on-board sound-card, you can use a Headset Buddy.  
Hello:  Where is the microphone jack for my XPS 8940?  I just want to plug in a regular 3.5mm 3/8 microphone into my computer to use.  I cannot figure out how to do that.  Can someone please advise if this is possible and if not, what do I have buy to get it to work.  Thank you.
XPS 8940 - Where is the microphone jack