They are different languages speaking, while in writting they are Entirely different , as Urdu writting style is like Arabic and Hindi, Malyalum , Bangali etc have Very differnt style of writting.
These languages Hindi and Urdu appears samilar because of the speakers mix up and now the Media advancement like TV serial , Bollywood movies etc.
Once my Indian Colleague was speaking purely Hindi , it was hard for me to understand.
You can see the difference in writting of Hind and Urdu from this site
www.bbcurdu.com and www.bbchindi.com here you can listen to the two different languages also in their pure versions.