Every "distance between two GPS coordinates" problem eventually reaches for the same formula: haversine. This calculator puts it in one place — enter two latitude/longitude points and get the true as-the-crow-flies distance between them, in whichever unit you need, plus the compass bearing to head at the start of the trip.
Why not just use the Pythagorean theorem?
Latitude and longitude are angles on a curved surface, not flat x/y coordinates — a degree of longitude covers a shrinking real-world distance as you move away from the equator toward the poles. Treating coordinates like flat Cartesian points, the way a simple Pythagorean-distance calculator does, gives badly wrong answers for anything beyond a very small area. The haversine formula accounts for the Earth's curvature directly, which is why it's the standard for any real-world lat/long distance problem — flights, shipping routes, delivery radius calculations, and more.
Great-circle distance vs. driving distance
The number this calculator returns is a straight-line, over-the-curved-surface distance — not a driving or flying route. Roads bend around terrain, property lines, and infrastructure; flight paths bend around air-traffic corridors, weather, and jet streams. Great-circle distance is always the theoretical minimum; actual travel distance is equal to or longer than it, sometimes substantially so for routes with few direct roads or flight paths.
What the bearing tells you (and doesn't)
The initial bearing is the compass heading at the very start of the great-circle route. Because a great circle looks curved on a standard flat map — the same reason flight paths on an airline map appear to arc northward on transatlantic routes — the compass heading actually changes continuously along the route. The initial bearing is only the direction you'd start out heading, not a single heading held for the entire trip.