Android :: How Android Handles Apps / Processes?

I've got the htc legend and its an awesome phone. I just wanted to ask to get an idea of how android works on relation to how it handles apps/processes etc. Using a task manager I see that tonnes of apps start when the phone boots. I see apps like app brain, msn talk,.radio, twitter app, stocks, footprints, maps and a whole plethora of others. What makes them start like this? What's the purpose? Do they have a negative impact on performance? It seems like all the apps listed see ones which have shortcuts on the home screen or related widgets. Is this how it works? Although they are listed date saree are they actually taking up system resources? And then there's ones like radio and stocks which I never use and don't have home screen shortcuts for, but they still start. Sometimes when mass kill apps (not processes) it sorta freezes for a while then has to reload the home screen. None of the apps I killed were essential ones, happens sometimes when I kill a bunch of third party apps. Would it be beneficial to kill most of the listed apps which apart on startup that I never use? I try to refrain from using the task manager but Coming from windows I'm just used to massacring all unnecessary processes and programs to keep everything clean.