2011 MacBook Pro monitor as a 2nd display for Windows 7 (Display port) laptop How can I use my 2011 MacBook Pro monitor as a 2nd display for my Windows 7 Lenovo Laptop (Display port)?
iMac's support what is known as Target Display Mode. However, this is limited to iMac's only, Macbook Pro laptops do not support this.

Alternatively, you can use your Macbook Pro as a secondary monitor by using an app such as Air Display.

Air Display is an app that will allow another shared desktop to display on it, over wifi.  However, if you are doing any critical photo, video or even games, you will find the lag to be very annoying.