Seth Godin has a great article at Seth's Blog on Taking a share of an existing Market compared to creating a new Market.
A similar idea you may have heard of is called Blue Ocean markets. If you know anything about sailing you will know that there are large amount of oceans that very few people have ever sailed over. These are the blue ocean markets.
Is your business creating a new market? I think it is just as important to understand the nature of your business as to be one or the other.