You have a laptop and want to use it as a second monitor to mirror or extend your Windows Desktop? Here are some options for the different Windows versions.
For Windows 8 and Windows 10 you can use the software Spacedesk, it is available for free as long as you use it for personal or non-commercial purposes and can be downloaded from the vendor website here:
Spacedesk has full support for Aero and can mirror or extend the Desktop to the second device. Spacedesk has to be installed on both devices.
For Windows 7 and earlier Windows versions there is a nice Freeware called ZoneScreen that is able to Extend- or Mirror the Desktop to another Windows Computer or a Mobile device. The drawback with this software is that it does not support Aero, so the Aero Desktop has to be switched off. It seems as if its development has been discontinued
ZoneScreen is available here:
If you just need a Desktop mirror (and not a Desktop Extension) then you could try a VNC software like RealVNC.