What is the difference between List and Electorate MP's in NZ?

In New Zealand the government uses MMP to decide the government and electorate MP's.

The Electorate MP's are the ones who represent people of a certain area. The List MP's are the MP's that are added in so that all the seats the party has will be full.

eg. if a party got 70 seats in the government and out of those 70, 40 were electorates then 30 list MP's will be present.