Alltop describes itself as an “online magazine rack” of popular topics. As with a magazine rack, you can browse through a variety of topics [i.e., blog feeds] and explore the ones that interest you in more depth. It's a pretty neat concept, and an extremely efficient one for exploring the many topics about which people write.
It does so by aggregating RSS feeds from popular resources. As Alltop explains on its About page, to help you "explore your passions by collecting stories from “all the top” sites on the web [... and then groups] these collections — “aggregations” — into individual Alltop sites based on topics such as environment, photography, science, Muslim, celebrity gossip, military, fashion, gaming, sports, politics, automobiles, and Macintosh. At each Alltop site, [it] displays the headlines of the latest stories from dozens of sites and blogs."