What is difference between desktop app and mobile app ??

Answer 1

Screen Sizes
Operating Systems
Device Capabilities
Hardware Components
Speed
Form Factor
Compilers
Binaries
Etc

The question is a bit vague.

Answer 2

One is on desktop and other is on mobile, does this quality as a difference ?

Answer 3

Utsav Parikh , are you asking what is required to make a desktop app into a mobile app and visa-versa? or something else, like, what are the coding differences and considerations when initially creating one?

Please give us more to go on, or get many answers that don't answer the question you intended.