What is the difference between "Network Unlock Code" and "SIM Unlock PIN"?

I bought an unlock code because my phone was asking for it (Enter PIN Code). I got the code and it does not work. It actually only allows me to enter 4 digits. Now, these guys are telling me that the code is for the phone's "Network unlock" not the "SIM unlock", and if I don't remember the PIN, I will need the PUK code from my network provider, so I feel that I paid for something I don't need. This is the first time I try to do this and I feel frustrated. I would really appreciate any help.