I've googled a lot about X button, that doesn't quit app, but simply close the window andnd haven't found a clear answer. I want to be able to know if open apps affect battery life or other resources.
Should I quit apps or close windows, letting apps work in background?
One of Mac's most confusing features is the red close button, which you think would shut down an application but often doesn't. When you press X button in the window - you actually close the windows related to the app, but it does not quit. Here's more information from Apple's support pages:
Close windows
When you close an app’s window, the window closes, but the app remains open. If you want to quit an app, click the app’s name in the menu bar, then choose Quit [App]. For more information, see Quit apps.
If you want to quit the app (release the memory, reduce CPU usage, etc) then there are several ways to do this:
Quit option.Applications>Utilities>Activity Monitor.You may also want to overload the default behaviour, and to make it like in Windows OS, but you need to use separate utilities (like RedQuits). There are a bunch of those utilities that close applications when you click the the red close button.
In overall, to answer your question about how it hits system efficiency, memory and battery: OS X handles it properly and does as much as possible to optimize efficiency, unless the Application itself is written poorly. OS X internally has a lot of optimisations like App Nap which help to keep background apps' effect on system resources to a minimum:
You don’t have to quit apps once you’re done working in them — OS X allows you to work with several apps open at once. Features such as Compressed Memory and App Nap keep your Mac running fast, and save power when many apps are open.
There's no real easy answers to this. It's going to depend on such things as how much memory your system has, what the apps are, how well are they written etc.
For example, if you have a small amount of memory and checking Activity Monitor shows little memory available, or a lot of swap space being used. Then you probably want to ensure apps are quit to make more memory available. Adding to the confusing around this is the fact that even though you quit an app, OS X doesn't remove it from memory until it needs the space for something else. This is why quitting and reopening an app is faster than the first time you started it. If the app is still in memory, OS X only has to start it again. Of course, in a memory tight situation, OS X automatically removes apps from memory which have been quit so that the memory can be available for something else.
Apps which you have closed all windows for, but not quit, can effect the system. It depends on what they do. So memory, CPU, battery can all be consumed. Again Activity Monitor is your friend in determining whether any of your apps are causing problems.
Generally speaking, if you have enough memory, and your apps are well behaved in terms of sucking on your battery, then whether you quit or close is not going to matter. Luckily, OS X apps are generally quite well behaved.
In general it is fine to just close the windows. If you are a software developer or otherwise have very special needs, you might need to quite the applications explicitly. However, in general - just close the windows.
The other answers to your question does not seem to take into account that the mechanism regards closing windows vs. applications was changed with OS X Lion. From that version and on, OS X will automatically quit non-used applications for you.
The feature is called Automatic Termination. It quits applications when the system deems it necessary, but only if you have closed all the windows belonging to the application and the application is no longer the active (currently selected) application. In addition, the application needs to opt-in to Automatic Termination - but most ordinary application do opt-in.
