Declarative: Specify the result you want.
Imperative: Specify (step by step) how to get the result you want.
Examples:
Declarative: I want a PC with the following code installed: X, Y and Z.
Imperative: First upgrade X, then install Y, then uninstall W, then install Y and Z.
Examples:
Declarative: I want a PC with the following code installed: X, Y and Z. Imperative: First upgrade X, then install Y, then uninstall W, then install Y and Z.