The differences are:
Render Size Maximum: 800x600
Render Detail Maximum: 1.0
Antialiasing Maximum: 3
Maximum Number of Rendered Populations: 3
Maximum Render Detail for Micropolygon Export: 0.1
You can find a full description here
http://www.planetside.co.uk/content/view/21/36/.
In addition, no access to the animation functionality that comes with Terragen with Animation version.
A great way to learn the product without the pressures of the more usual 30 day demos that most software companies provide!