Well - welcome back!
Task management...
OK, so here's how that works.
Some apps will actually exit - but most simply get put into a holding pattern - literally, they're put to sleep by the system.
Unlike other phone operating systems, Android is basically a form of Linux, so is a unix and in that world, apps have always been all about building blocks.
Jobs get done (maps, music, email, web, whatever) by stringing smaller building blocks together, rather that by running large hogs.
Underneath the app layer are the services. So - take your music player. If you back out of it, Android's cool with it, will let it sleep - but the services it uses keep going, so your music still plays. To stop it - stop the music.
Other apps have other paradigms - play some music vid in your browser, back out of that - the music stops.
A lot of cooperation is going on under the hood, controlled by Android, so stopping apps is often not a great idea (that said - on occasion, one will lose its mind or seem to get bogged down - ok to stop it when that happens using the Application manager and then re-start under those conditions).
Now - what really uses power and memory are apps that attach to a lot of services.
And carrier-installed bloatware is typically famous for that - hence the advice to root and remove that stuff. Otherwise, thanks to abuse of system-safe features, you kill them and they just come back.
If you'd like to see a profile of your running phone - along with how little memory each app and building block takes - check out System Panel Lite, free in the Market. Also OK for killing the occasional app -
when absolutely necessary.
But yeah - your takeaway here:
task killers are bad, mmmmk? 
