You do not 'close' windows in Android the same way you would in Microsoft. Instead: you let Android partially close (hibernate) and fully close your windows for you.
How Android Manages the Partial and Full Closing of Software Windows:
If you no longer wish to use an Android program, you simply leave the program by doing any of the four options:
1) tap the 'back' arrow button
2) navigate to 'home'
3) launch a new program,
4) or use 'recent apps' button to launch a previous program.
As soon as you leave a program, and that program is not doing anything, then the program 'hibernates'. Hibernation is a partial close, where it is moved from system memory into storage memory. This hibernation frees up system memory, yet still remembers the state and configuration of the hibernating software.
The benefit of this hibernating-type closing is that 80% of the time, you can return to the exact same screens when you relaunch the program. Not all Android programs strictly follow this, but this feature is very useful nonetheless.
So, in short: you don't personally close windows in Android. You let Android close windows behind you as you navigate.