What is the difference between a Sher, a Ghazal and a Nazm?

Urdu/Hindi/Hindustani poetry are of two forms

A Sher is a couplet. Its a very short verse consisting of very few lines. Plural of sher is aashar

Ghazal is a poetry where in there is no specific central theme. Each couplet(pair of lines)/Sher can be taken out separately from the poem and interpreted independently. Each paragraph in a ghazal can be a separate aashar/Sher. Each sher in a ghazal may or may not have any related to the previous sher/aashar.

Here is a example of Ghazal

http://untamed-life.blogspot.com/2007/07/sher-by-ghalib.html

A Nazm has a central theme. A line/couplet cannot be taken out of the Nazm and interpreted independently. Each line/paragraph is a continuation of the previous. Taking out a line/couplet would essentially destroy its meaning.

Here is a example of a Nazm

http://untamed-life.blogspot.com/2011/01/baat-niklegi-to-phir.html