Sourdough bread is quite lovely and also quite different.

Soda bread does not have any yeast and relies on the action of the baking soda to make the bread rise. http://www.ibmpcug.co.uk/~owls/sodabred.htm

Sourdough uses a yeast "starter" which is well described in the following website http://joejaworski.com/bread1.htm

The great thing about Sourdough bread is, for me, that it doesn't have any added sugar. When I lived in New York for a while in the early eighties I spent ages trailing round shops in search of bread that was not heavily manufactured. I didn't discover any bread that did not taste sweet. I began to wonder if it was possible to make bread without sugar (I checked with a baker back here, the yeast works quite happily without sugar). I was delighted when I went back to the USA in the early nineties to discover the popularity of sourdough bread. I'm sure it was there all the time, I was just looking in the wrong places!