Microsoft is attempting to force people to upgrade to Windows 10. Admittedly there is no charge for the upgrade if done before July 2016 but there are numerous reasons why you may not want to take advantage of this 'free' upgrade.
From what I have seen Windows 10 loads significantly slower than Windows 7. It also does not give you a choice about installing upgrades which both can use up significant bandwidth and has the potential to break your system if it turns out that the upgrade is incompatible with your hardware. The bandwidth usage alone might cost you a large amount of money and perhaps wipe out your ability to connect to the internet for any other purpose if you are on a slow connection with a small download limit. Presumably at least some of the issues people are having will be fixed before the period for upgrading 'free' expires.
Microsoft is installing a "Get Windows 10" button in the taskbar to supposedly make it easier to upgrade. If you believe that Microsoft is going to eventually fix Windows 10 you can hide this option temporarily by right clicking on the date/time at the end of the task bar and selecting "Customise Notification Icons" from the menu. In the window that appears scroll down to find the GWX application and change the "Behaviors" value to "Hide icon and notifications". This option leaves the update option still running but hides the button so that you don't accidentally click on it. If you later decide to upgrade then simply change the "Behaviors" back to "Show icon and notifications" and the button will reappear.
To get rid of the option more permanently we can delete the program itself. To do this you first need to stop it running. To do this either right click on the taskbar and select "Task Manager" or press CTRL-Shift-Esc. On the Processes or Details tab (depending on which version of windows you are running) locate and select the GWX.exe entry and then press the delete key. Next press win+R to open a run box and type in 'tasks' and press Enter. Now go to Microsoft\Windows\Setup and delete both the GWX and GWXTriggers folders located there (you may need to change permissions to be able to delete everything). The final step is to locate and delete C:\Windows\System32\GWX and if you have a 64 bit operating system C:\Windows\SysWOW64\GWX. This means that this application is now shut down and completely removed from your computer.