I'll give this my best shot, but you'll need to do some reading on your own.
The flight sim world rests on top of "mesh".There is no
ONE mesh. There are meshes for different areas of the world.
To some degree, the quality, or depth, of the mesh defines how well/detailed the mountainous scenery looks.
If you fly the big tubes high and fast, the quality of mountain detail us pretty much unimportant.
If you like low and slow flights in mountainous, or island, areas then the quality & detail of those mountains can make your day.
But if you like flying around Florida, Indiana, Kansas or other "flat" areas then the standard mesh within the sim will more than suit your needs.
There are good reasons for this, but essentially the best mesh's will be in North America and Europe. After that it's anybody's guess. Australia? Asia? Africa? Antarctica and small out of the way places will be dead last.
This is one of the Canary Islands with the standard FSX mesh. It was picked as a "worst" example of FSX mesh.
And these are shots of the same island after FSGenesis.

Now you can see why I started off with mesh.
There are two commercial mesh's.
FS Global Ultimate - more than adequate, but VERY expensive if you want to cover the whole world.
FSGenesis - World Terrain Mesh - the best bargain around, $25. This is what I bought and I have no regrets. It's slightly lower detail in some areas than
FS Global Ultimate but if you buy some of the Orbx regions then you'll automatically get higher resolution mesh with the region.
Now there might be a better option, and it's free -
FreeMeshX. According to them, their mesh is made from a newer, and higher resolution, NASA survey.
One thing to remember, NONE of these will be perfect.
I'll point you to a simple, but good, explanation of how it all comes together -
http://www.orbxsystems.com/forum/topic/74347-the-definitive-guide-to-ftx-products-the-go-to-source-of-ftx-information/?p=681770.
Download the FTX Definitive Guide v2.pdf guide and go to page 13.
When you're done with that, check out
Blue Sky Scenery for some of the best photoreal scenery around. AND it's free.

