The classic hierarchy ("URLs and URNs are distinct subsets of URIs") wasn't as neat and useful as originally thought, so "it's just URIs now, with their own scheme-specific qualities" is the modern thinking, as reflected by this passage from RFC3986.
The modern answer to "What's the difference between a URI and URL?" is thus very nearly 'Mu.'
The classic hierarchy ("URLs and URNs are distinct subsets of URIs") wasn't as neat and useful as originally thought, so "it's just URIs now, with their own scheme-specific qualities" is the modern thinking, as reflected by this passage from RFC3986.
The modern answer to "What's the difference between a URI and URL?" is thus very nearly 'Mu.'