What are segments, audiences and comparisons in GA4, and how would you use them?

All three isolate a subset of data, but they behave differently.

• Comparisons — applied in standard reports to view side by side, such as mobile against desktop, or one country against another. Quick, temporary, and limited in complexity.
• Segments — built in Explore, and far more powerful. Three types: user segments (everyone who ever purchased), session segments (sessions from paid search), and event segments (specific events matching conditions). Segments can use sequences — users who viewed a product then added to cart within an hour — which is where the real analytical value is.
• Audiences — segments made persistent and exportable. Once defined, GA4 starts collecting members going forward, and they can be pushed to Google Ads for remarketing. Crucially, audiences are not retroactive: they only populate from creation, which is why you should define them early.

How to use them analytically: aggregate numbers hide almost everything interesting. A 2% overall conversion rate might be 5% on desktop and 0.4% on mobile — the average tells you nothing actionable, and the segmentation tells you exactly where to work. Comparing converters against non-converters to see what behaviour differs is the single most productive analysis pattern.

Note: The 5 concurrent segments per exploration limit is worth knowing, as is the fact that audience membership counts towards remarketing only after the audience exists.