Android :: Application Without Main Activity?

Is there a way to manage application activities by hand, like this: user activating an application from menu, it does some initialization, then creates some activity (is it necessar y to declare all activities in the application manifest?), and listens to it's events. On some event application decides to close one activity and open another - so it contains all the application logic. Didn't found anything like this in examples, they all have all the logic in the activity classes. Maybe I need to user Services? (Maybe I don't understand right, what an activity represents. For me it's like window in windows, or Displayable in j2me) I'm very new to android development, trying to understand the basics.