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What is a junior gold stock?

A junior gold mining stock is a public company engaged in identification and development of precious metal resources. The company can hold multiple resources even districts over many different countries.
A junior gold stock definition from PDAC (Prospectors and Developers Association of Canada)
Mining companies are defined largely by the way in which they derive their revenues. A senior producer or operator generates its revenues from the production and sale of the commodity it is mining. A junior mining company has no mining operations and is essentially a venture capital company. It must rely almost entirely on the capital markets to finance its exploration activities [I say "almost entirely" because some juniors derive their financing from private sources]. There is another category: mid-tier producers. These are generally junior companies that have decided to go into production on properties that they have discovered.

There are two major models of junior miners follow (although alot of companies incorporate parts of both), the project generator and the resource developer models.
Project generator
A project generator will typically purchase large land packages, significantly identify and 'prove up' a resource and will either sell the property outright or joint venture (JV) with a larger company.
Resource developer
A resource developer may still buy large land packages but will intend to identify and 'prove up' a resource then to build the producing mine. They try to do this will minimal dilution to the stock holders but may still JV a small percentage of the mine. At the point when the first mine goes into production the company becomes and mid-tear gold stock.

Reference: PDAC (Prospectors and Developers Association of Canada)