GIS is a fast evolving technology used to capture, manage, analyse and display information in its geographic space. It allows us to understand and visualize data in a variety of ways.

Here are some more GIS definitions:

Is it a big industry?

Geographic information systems are one of the most widely used technologies across the globe, and GIS professionals are in high demand. GIS careers exist in almost every imaginable discipline and every industry. According to a GIS research firm, GIS  grew worldwide 10.3% in 2010 to US$4.4 billion with a forecast of an additional 8.3% growth to almost US$5 billion  in 2011.

In this market, Esri is one of the leading companies. But many players exists within specific industries and pockets, so a well-rounded education and experience is key to succeed.

What does a career in GIS look like?

A GIS career will take you on a journey to interpret and visualize spatial data, to uncover patterns, trends and relationships with leading edge software tools. Working on GIS projects requires attention to detail, technical mastery and good analytical skills. It also demands you to be versatile, being able to go to antartica to analyse data, map events inside a command and control emergency management centre or manage millions of assets records and images in utility networks.
A good overview of career pathways can be found here:
The Futureintech website has some great information about working in the IT industry including profiles of people working in GIS:
GIS Geography covers some common GIS career questions and insights:

What do I need to learn?

Simply understanding how to make a map in a GIS software program isn’t enough; also being able to use something like Goggle Earth will not cut it. Spatial statistics, geoprocessing, cartography, database management, software programming, web map services, and project management are all critical components of a well-rounded GIS education.  Most employers are looking for GIS professionals to be able to walk into a situation, know how to understand the needs, and deliver on the geospatial products and solutions.
Most New Zealand universities offer undergraduate GIS papers
An excellent New Zealand postgraduate program for GIS is the Masters in Geographic Information Science offered by the University of Canterbury, the Auckland University of Technology and Victoria University.
Most of the advanced GIS job openings are aimed towards people with programming skills or experience with specific product lines or solutions. The in-demand GIS professional will need to be well versed in GIS software and programming languages.  Also new technical certifications are coming into play in the Esri field, so make sure you check these out.