Your pic is doing all ethernet stuff like DHCP, IP, UDP, TCP (They are network layers which are described better in microchip document named AN1120) ENC is a tool that is need to arrange these stuff. An example is here. An ethernet packet is come to ENC register. ENC is produce an interrupt to pic and pic (you) is reading that packet.
However, these network layers and packets are very complicated. There are lots of datas need to be read or be written to ENC. So, TCP-IP Stack is playing a huge role here. TCP-IP Stack is nothing but source codes which simplifies all packets of layers. Microchip TCP-IP Stack is free and been described in microchip document AN833.
Proteus has a simulation of ENC28j60 perfectly. (However i cannot set to work it unfortunetly)
If you want a quick start, i suggest you to look proteus Explorer16 ethernet board simulation. If you good at software, you can change the c codes of this simulation and make your own code (or made your own board with proteus)
When i finish it, i will post it here