As a terragen artist, I have come across quite a few questions which have been asked by various people wishing to know more about terragen. So, I've decided to list a few of those questions here.

What is Terragen?
Terragen is, in essence, a digital scenery/landscape generator which was designed to create photo realistic landscape images, as well as animations. However, it is also a "work in progress" program that is continuously being updated.
Example image made in Terragen:

As It All Ends by *Druology

Is there a demo version? What are the limitations
Yes, there is a demo version available - and it's perfect if you want to use it to find your feet in using terragen. However, you are limited to using the smallest terrains which lack some detail, as well as you have a limited image "rendering" size which may distort your final image.

Can I buy it?
Of course! You can purchase your copy of terragen over on the homepage: Planetside The registry key will be emailed to you within a few days, depending on the say you register. Once your copy of terragen is "unlocked" you can render images up to whatever size you want, as well as use terrains above 513x513. The bigger the terrain you use, the more details you will get.

What's the difference between Terragen and Terragen 2?
Terragen is essentially Terragen 2's (or TGD's) predecessor. Terragen 2 is much more advanced with it's image rendering capabilities, and is able to render much more realistic images.
Terragen vs Terragen 2:

Aldranon of Winter by ~HorseLips (Terragen 9.43)

Bright Angel Canyon by ~Buzzzzz (Terragen 2)
As you can see from the second image, Terragen 2 has "superior" rendering capabilities. Terragen 2 also has the capabilities of adding in foliage and structures pre-render so that there's less to postwork into the image!

DEM files - what are they?
A DEM file is a simple, regularly spaced grid of elevation points, and the most common kind of DEM files come from the USGS (U.S. Geological Survey). Once converted into a ter file (for use in terragen) you are able to generate "real" landscapes within both Terragen and Terragen 2
Example of used DEM file

St Helens by ~dunkst3r
Mount St. Helens is the single most used DEM file - just take a browse through some of the galleries here But the results as to why is pretty easy to see.

Texture Overlays
Yep, terragen fully supports the use of texture (or image overlays) within the program. This is used to generally add more realism to the overall image. However, you do need the necessary plug-ins to use this function.
Example of Image Overlays

Dream in Black by *Ladyofspira

Ruins of the Third System by ~HorseLips
As you can see, the plug-ins allow for the expansion of terragen's capabilities. These are available for download over on Planetside: Resources

Is there any groups or tutorials here on deviantART
Indeed there is, there is quite a decent sized terragen community here on devART. Though, not as active as it once was - artpacks and club activities do still take place in some of the clubs.
Clubs
(inactive) (inactive) (inactive)(inactive) (inactive)
Tutorials
A list of terragen related tutorials is available here:
Terragen: The tutorials
Also, be sure to check out the following websites for help and further advice
Terranuts
Christian Fly On The Web
TerraProject
Terragen Landscapes

If you have any further questions and queries, feel free to send me a note or leave a comment

Until next time!