Differently Abled:

Person A has some skills that person B doesn’t have. Person B has some skills that person A doesn’t have. Society respects the existence of both of them, and considers both of their skill sets to be useful. Society does not require either one of them to use skills that they don’t have.

Disabled:

Person A has some skills that person B doesn’t have. Person B has some skills that person A doesn’t have. Society treats Person A’s skills as default skills that everyone’s supposed to have, while considering Person B’s skills to be useless. People who don’t have Person A’s skills are expected to learn those skills or be left behind, while people who don’t have Person B’s skills are given the tools to not need those skills. 

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“Differently Abled” is not just a nice way to say “Disabled”. It erases the fact that society actively causes certain people to be disabled.