You first get Residence, along with two years' travel conditions, which mean that you can come and go as and when you like.
After two years, the travel conditions lapse - you're still okay for living in NZ, but to have the right to travel and come back into NZ, you need EITHER to renew the travel conditions (and how long you can renew for depends on how much time you've spent in NZ over the previous two years), OR show that you have a commitment to NZ, and you will then get Permanent Residence, which gives you the right to travel and come back as much as you want, forever.
Demonstrating commitment to NZ can be done in various ways, but the most common one is to have spent 184 days or more, of each of the last two years, in NZ. Here's the link to the others.
http://www.immigration.govt.nz/migra...quirements.htm
There's more detail on the various possible situations given in the links on the sidebar on that page. As almost always, the INZ website has the answers.