A sporecast is a "grouping" of content picked by players. If you "subscribe" to a sporecast, the content in this group is more likely to appear in your game. I subscribed to a "cartoon characters" sporecast and met the Daffy Duck race in my game (the voice was...eerie!).
Making sporecasts is pretty easy, that is, once they fix the sporepedia up. I'm looking forward to sporecasts based on plants.