Laptop Connections

I have changed many hard drives in desktops but this is my first failed attempt at a laptop. Can someone provide some insight. I have a dell Inspiron 5100 and purchased what I thought was a replacement drive. Upon trying to changeout, It became apparent that the connection was different from what I had purchased. The Dell drive has like a flat blade that inserts into a slot. The one I purchased had pins in the back much like the Desktop drives I am used to replacing. Would someone be so kind to inform me what type of connection I am looking for on the proper 2.5" drive. Or better yet would someone be so kind as to go to Newegg and send me the link for a 30-40 gig drive that would work. I will definately have to send this one back. I did lok at caddys but could not determine which one would work. It also looked like there was not enough space for a converter cable. Thank you very much and Merry Christmas.

James