Unlike the iPhone, the Droid can run more than one app at a time.
iPhone users making the transition, like John Gruber at Daring Fireball, sometimes say things like this: “I have no idea how to quit an app other than reboot.”
Don’t despair dear Droid newbie, Droidie is here to help. 
Personally, I use Advanced Task Killer app to clean out the apps I don’t want running.
It’s pretty easy to use. Uncheck the apps you want to keep and click the button at the top of the screen and poof they go to heaven until you run them again.
If you want to quickly see which apps are running, hold down the Home key at the bottom of the Droid.
DROIDIE 
January 16, 2010 at 12:05 pm
Words fail me.
January 16, 2010 at 12:11 pm
Actually, a long press on the home key shows the last 6 apps you have accessed. App killer shows you what is running.
January 16, 2010 at 12:19 pm
“If you want to quickly see which apps are running, hold down the Home key at the bottom of the Droid.”
Actually, that’s a list of recently-used apps. For example, right now the only ‘recent app’ on the list is The Weather Channel; Advanced Task Killer tells me Google Voice and K-9 Mail are running in the background, but not TWC.
January 16, 2010 at 3:29 pm
Well, the most recent tasks/apps you can see in the recent-task-switcher don’t mean at all that these apps are still running.
Also, in particular on the Droid a task killer is not necessary, if you are not a tweaker.
When the OS needs more RAM it just kills one of the processes/apps that are not in the foreground and haven’t been used in a while. This way it is fast to switch back to an app if it is still in memory, and still possible for apps that are not in memory anymore, because the UIs save their state, so that they can be resurrected in a way that is transparent for the user.
January 16, 2010 at 9:38 pm
I used Task Manager for a while, but every time I looked, it showed that the CPU was not pegged. Not sure about RAM, but as noted, Android seems to handle that. Although I have experienced Android lagginess, it never seemed to make a noticeable difference when I would use Task Manager to clean up things. My biggest performance problem lately is getting the home screen to paint–that sometimes takes as long as 5 seconds (I am using the native home screen, no Home apps).
In the event you have a really buggy Android app, killing it is obviously crucial.
January 17, 2010 at 12:54 am
Erik, just to add to what you said, if an app in the foreground gets slow and doesn’t respond within 5 seconds to user input, Android will show a dialog “application not responding. wait/force close”.
Regarding the homescreen? Do you use the Droid?
In particular on the G1, which is very memory constrained, the home screen app/process might get killed when RAM is urgently needed. When you get back to the home screen then it needs to be restarted, which takes some time and widgets add to that.
January 17, 2010 at 7:30 am
Hi Mariano. I have a myTouch 3G, not the Droid. I believe they have the same specs–256 RAM, vs the 192 in the G1. I follow your reasoning, and the problem is somewhat correlated to how long the phone has been on, in that a fresh re-boot solves it for at least an hour or two. But it happens too much to be just an occasional fluke; could be a badly-behaved app, but if so, I haven’t found it yet. I have sworn off widgets, and as many “TSR” things as possible.
I actually did a Master Reset 2 days ago (what a pain!) and it doesn’t appear to have made a significant difference, either.
January 17, 2010 at 12:32 pm
Erik, I said before that a task killer is not necessary, especially on the Droid, and I stand by that. But it was also a simplification and for memory constrained devices the story might be a little bit different.
I think there is one thing where a task killer could do better than the OS. The OS just nows on a general level what is important: The app currently in the foreground is more important than the one the user was using a minute ago, which is still more important than a background task started an hour ago and which took 12 MB of RAM for example … But this is a generic, heuristic approach.
I never actually used a task killer, but from what I know you can define which apps are dearer to your heart than others, so you can give input to which tasks you are more likely to get back, giving the task killer the chance to more aggressively kill the processes/apps that are not that dear to you.
Having said that, let’s be honest, your not so special after all 😉 Nobody wants to wait on their home screen, so the OS should give it a high prio. And I’ve heard about a “lock/pin homescreen” or something like that option on cyanogen mod. From the naming I would guess this is something exactly like that. You give away a couple of MBs to keep the home screen in memory at all times.
Btw. I used my G1 for a year and never used a task killer and my girl friend’s G1 is even snappy. Maybe that’s because she just doesn’t install a lot of stuff.
And this whole task killer thing is exactly why people like gruber, I guess, perceive Android as too complicated.
March 21, 2010 at 9:49 am
You guys are missing the boat on this. Task Killer is not necessary for phone functioning. It is essential for battery life. If I leave my navigation via Google maps on, it will kill the battery fast. It is very quick to do and is no more annoying than going thru the process of waking up your phone.
January 17, 2010 at 1:39 pm
Good points. Yeah, I’m pretty sure that everybody wants a response home screen. “Speed of access”, in various respects, for key apps is really important, I have some posts on the subject:
http://mondegreen2.blogspot.com/2009/11/gestures-for-android-speed-of-access.html
http://mondegreen2.blogspot.com/2009/11/speed-of-access-is-key-for-smartphone.html
(I have not felt the need to root my phone at this point, just noting for full context.)
At this point, I am hoping that the Android 2.x upgrade–whenever that comes–may fix my Home screen problem. At this point it is less than a showstopper, but a bit more than a minor nuisance. And it really is the Home screen, because when I do the long-press on Home to get the last 6 apps, that is perfectly responsive.
January 18, 2010 at 3:27 pm
I do like having ATK, but I rarely use it.
I hear a lot from former users of it, and from fellow developers, and I’m pretty sure a lot of users are shooting themselves in the foot.
If you kill a task, and then open it again before you open 6 different apps, you’re likely not doing yourself any favors.
Android aint perfect. I like to think it fails gracefully 🙂 But what it is a very good at is doing automatically what ATK does manually.
But some apps are buggy, and buggy apps need to die completely when they bug out, so I always install ATK, and sometimes use it.
January 28, 2010 at 4:41 pm
I use “Better Terminal Emulator” with su and kill -9 from the linux console. It just feels better that way.