The difference between croissant and Danish pastry dough is in the ingredients.
The puff pastry dough is made up of flour, water, and salt; croissant dough is prepared with flour, sugar, yeast, salt, and milk.
While Danish dough includes the same ingredients as a croissant dough but adds eggs to the mix.
The dough is mixed together until it becomes a sticky mass.
Classic Danish pastry is crisp, tender, light and slightly flaky and is simply referred to as "Danish."
Genuine Danish is true pastry, falling somewhere between croissant and brioche, but contains about twice as much fat as croissant dough and American-style Danish.
Plus, it contains an egg, which croissant does not.
A Danish is leavened with yeast, as opposed to puff pastry that is raised by steam.
Danish pastry also contains sugar, hence with a composition of sugar and yeast, the high sugar content in the dough makes a Danish better catered to sweet creations, such as the commonplace 'Blueberry Danish' we see in local bakeries.
Croissants are made of laminated dough.
There are two main differences between classic puff pastry and croissant dough: Puff pastry doesn't contain any yeast while croissant dough does.
To make puff pastry, you need to make 5 single turns, and to make croissants, you need only 3 single turns.