What exactly is a chowder?
According to Food Network and chowhound.com, chowders are thick, usually cream- or milk-based soups with chunks of vegetables and/or fish. A cream soup is generally pureed, a thinner consistency and smooth, with no chunks of food.
Food Network also notes the regional aspects, in that the Northeast seafood-based chowders are more common, while further inland meat-based soups are more prevalent.
http://www.foodnetwork.com/how-to/chowder-and-soup/index.html

Chowder's roots are in the Northeast, with the most popular being New
England Clam Chowder. It has been said that the soup took its name
from a type of French cooking vessel, the chaudiere. But the French
can't claim responsibility for this truly is an American tradition.
Customarily, chowder included onion, potatoes, and cream. Nowadays,
not all chowders adhere to these guidelines. New England Clam Chowder
is sometimes made with milk, and Manhattan Clam Chowder doesn't have
any milk or cream, but has a tomato base instead. As chowders pop up
across the country, they have taken on many different ingredients but
most people still expect a chunky, creamy soup. So whether it's corn
chowder or seafood chowder, it will not be a smooth puree and it will
not be thin and wimpy.

PreparedPantry.com describes chowder attributes:

A chowder is a thick, rich, chunky soup usually made with a white
sauce base though Manhattan chowders have a tomato base.  Originally,
it was a fishermen’s stew made with seafood but today, vegetable
chowders are common such as corn chowder and potato chowder.
Chowders typically have five parts:
The vegetables or seafood, the focal ingredient of the soup.
Ancillary ingredients which may include cream, diced onions, bacon, or herbs.
The cooking liquid—usually broth or stock.
A thickening agent—usually cornstarch, flour, or potatoes.
Seasonings including salt and pepper.

http://www.preparedpantry.com/Recipes/How-to-Make-Chowder.htm