From what I've observed with the 'recent apps' button, this list is only the recent apps that you've had open. Most apps (if I've been in them and just hit the home key or switched to a different app) seem to take me back to where I left it if I go back to it within a few minutes. However, if it's been longer than 10m or so, it seems to have closed out and opens like it is starting fresh. So even if you have 20 apps in the recent apps list, not all of them are actually running and taking up resources.
I don't know if it's an app specific setting or Android setting. Games seem to close faster than something like GVoice or GMail.