In a nutshell:
An imperative language specfies a series of instructions that the computer executes in sequence (do this, then do that).
A declarative language declares a set of rules about what outputs should result from which inputs (eg. if you have A, then the result is B). An engine will apply these rules to inputs, and give an output.
A functional language declares a set of mathematical/logical functions which define how input is translated to output. eg. f(y) = y * y. it is a type of declarative language.