It's much like the difference between "how" and "what". SQL select statements are my favored example of declarative: you simply "declare" what you want ("all rows from this table with these values in those columns") and it's up to an engine to actually figure out how to do that for you. In an imperative language like C you could produce the same output, but you'd be specifying how, not just what: a for loop to iterate over each row in the table, if statements to compare values, etc.