In Google Reader feeds are organized by folders. (Just to confuse matters they also have something called tags. But don’t worry about that.)
You’ll see the feeds in the left hand pane, with some under the default news folder:
Click on that, and you’ll see a list of the most recent posts from that website or blog:
You’ll notice that if you click on a feed you’ll see only posts from that feed:
It’ll be clear from the title at the top what you’re looking at. In the above case, a selection of pieces on computer security from Topix.net.
Click on the folder, however, and you’ll get all the feeds within that folder, usually in chronological order (most recent at the top):
Here you’ll see that the title at the top is the name of the folder. A new column will appear on the left hand side of the pane, listing the details of the feed itself
As you gather feeds, things will quickly get out of control. So you may want to organize them via your own folders.
You can put a feed in as many different folders as you want. You can have as many folders as you want too.
More reading:
Google on the difference between folders and tags
Google’s guide to folders.