The main (in most cases) difference between free-range and factory-farmed eggs is that the birds of the former type are permitted to roam freely within the farmyard and kept in sheds or henhouses only at night. However, not all countries have legal standards defining what free range means. For example, the U.S. Department of Agriculture has no standards, and allows egg producers to freely label any egg as a free-range egg.
Cage-free egg production includes barns, free-range and organic systems. In the UK, free-range systems are the most popular of the non-cage alternatives, accounting for around 28% of all eggs, compared to 4% in barns and 6% organic. In free-range systems, hens are housed to a similar standard as the barn or aviary. In addition, they have constant daytime access to an outside range with vegetation. In the EU each hen must have at least 4 square metres of space.
For more information visit http://en.wikipedia.org/wiki/Free-range_eggs