Okay, I'm on Twitter now. :-D You can find me @MarionSipe.
So, today, I'm going to ramble about geography in worldbuilding. When it comes to worldbuilding, geography is a complex subject. One with so many variations and factors that it would be impossible to cover them all, even if I did know them all. However, there are some basic things that can help you shape your world, and shaping your world will help to shape your cultures. However, since I am not known for my brevity, I'm going to be taking these basics one by one. :-D
I want to add here that these are only basics; everything is affected by its own particular and specialized environment. When building a world, sometimes you want to stick to what is easily communicated, but sometimes you want to create something unique and beautiful. I don't think the "rules" should necessarily stop that process, but if they can be worked into it, well... So much the better, in my book. :-D What do you think? How closely do you "stick to the rules" when worldbuilding?
Coast lines are shaped by erosion. Sand is made up of rocks on the shore that have been pulverized, so the sand on your coastline will match the most common rocks and mountains there. If your coastline has basalt cliffs, the sand will be black, etc. Because different types of rock react differently to water, some sections of coast will erode more quickly than others. This can produce inlets, bays and sea caves.
A nation's coastlines are vitally important, especially if your nation trades with others or the people within it travel. But there are other factors to consider as well. Take the Great Barrier Reef as an example. This environment sits on a shelf of stone (continental shelf) which reaches out from Australia. It provides a home for numerous species of sea creatures because it is underwater, but close enough to the surface to receive sunlight. Coral thrives there, creating a couple thousand individual reefs, and provides the basis for a large variety of life, from sea anemones, to crustaceans, to fish, octopuses and any number of other creatures.
This kind of sea environment also affects the culture of the people who live there. It may create a large source of food of many varieties, but it is also a source of dangers. These effects can manifest in your world in a number of ways. For instance, because seafood could be so readily available, it might be considered 'common,' with the harder to catch or harder to eat items becoming 'delicacies.' Poisons derived from the sea creatures may be the most common means of murder, or because these toxins may exist in so much of the sea life, the peoples on the coast could even be immune to them, having ingested them in small amounts all their lives.
When creating a world's geography, I think the most important question you can ask is, "How does this affect everything else?" Because it does, in some way or another. Every change to the landscape creates a resulting change in the cultures, animals and other parts of the landscape.