Chromatography separates a mixture into its individual components by forcing them through a stationary material while a separate mobile phase carries them along, and because each component in the mixture interacts with that stationary material by a genuinely different amount, they travel through it at different speeds and gradually separate out. The same underlying principle covers several specific techniques depending on what the mobile phase actually is, a liquid in HPLC, high-performance liquid chromatography, or a gas in GC, gas chromatography.
One mechanism, different retention times, drives every chromatography variant
Every chromatography technique relies on the same core mechanism: a stationary phase the mixture is forced through, and a mobile phase carrying it along, with each component's own specific affinity for the stationary phase determining how long it takes to pass through, its retention time. A component that interacts strongly with the stationary phase moves through slowly, while one that interacts weakly moves through quickly, and that spread of different retention times is exactly what separates an initially mixed sample into its individual components by the time it reaches the far end.
The right variant depends on what the sample itself can survive
Gas chromatography needs a sample that's volatile and thermally stable enough to be vaporised and carried through the column as a gas without decomposing, which rules it out for many larger or heat-sensitive molecules. HPLC instead pumps the sample through as a liquid under pressure, handling non-volatile or thermally unstable compounds gas chromatography simply couldn't survive. Thin-layer chromatography, TLC, is the simplest, fastest and lowest-cost version of the same underlying principle, typically used for quick qualitative checks rather than precise quantitative measurement.
What we're still unsure about
That every chromatography variant relies on the same differential-retention-time mechanism, and that the right variant depends on what a given sample can physically survive, are well established, uncontroversial analytical chemistry facts. What's genuinely worth flagging is that chromatography separates a mixture's components without, by itself, identifying what any separated component actually is, a chemist typically has to pair chromatography with a separate detection technique, mass spectrometry especially, to know what actually came out at a given retention time, since retention time alone isn't fully reliable proof of identity, different compounds can coincidentally share very similar retention times under the same conditions.
This sits inside Chromatography (HPLC, GC, TLC), one of seven topics in Analytical Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.