The one-sentence answer
Session replay reconstructs the page from structured DOM data; screen recording captures the page as pixels in a video file. That single technical difference cascades into very different use cases — debugging real user bugs needs the DOM (so you can inspect elements + correlate with errors + generate tests); recording your own screen for an explainer needs pixels (because you're showing OS chrome, mouse cursors, and multiple apps).
How each one actually works
The mechanics matter because they decide what you can do with the recording later.
Session replay (Relyv, Hotjar, FullStory, LogRocket)
A JavaScript SDK loads on the page, takes an initial snapshot of the DOM tree (every element, every attribute, every text node), and then subscribes to a MutationObserver to capture every change over time. Network requests, console logs, click events, and form inputs are recorded separately and timestamped alongside the DOM mutations. The "video" is reconstructed in a sandboxed iframe at playback — the viewer rebuilds the DOM from the snapshot and applies mutations at the original wall-clock timing. File size is tiny (~30 KB per session, gzipped) because you're storing instructions, not pixels.
Screen recording (Loom, Zoom, OBS, QuickTime)
An OS-level recorder captures the screen contents frame by frame, encodes them as H.264 or VP9 video, and uploads the resulting MP4/WebM file. The video is opaque — you can play it, scrub it, share it, but you can't inspect any element, run dev tools, or correlate with backend events. File sizes are large (5–50 MB per minute at 1080p; more at 4K).
The differences that decide which to use
Seven dimensions where they diverge:
- <strong>Fidelity:</strong> Screen recording wins on visual fidelity (captures everything — OS chrome, mouse, system audio). Session replay wins on data fidelity (captures the DOM exactly, plus network + console).
- <strong>File size:</strong> Session replay is ~1000× smaller. A 5-minute session is ~30 KB vs a 5-minute video at 1080p is ~50 MB.
- <strong>Searchability:</strong> Session replay is queryable — you can filter for "sessions where users hit a 500 error on /checkout in the last 24 hours." Screen recordings are opaque files; you can only search them by title or transcript.
- <strong>Dev-tools access:</strong> Session replay lets the viewer open browser dev tools inside the replay iframe and inspect any element. Screen recording does not.
- <strong>Privacy:</strong> Session replay can mask PII on-device before upload (using regex, attribute selectors, or on-device LLMs). Screen recording captures whatever is on screen — masking after the fact requires re-rendering the video.
- <strong>AI workflows:</strong> Structured DOM data feeds AI summarizers + bug-ticket drafters + Playwright test generators. Video frames are harder to summarize accurately at scale.
- <strong>Recording trigger:</strong> Session replay auto-records every real user session in production. Screen recording is intentional — a human clicks "Record" first.
When to use each
They're different tools for different jobs. Most engineering teams need both.
Use session replay when…
A real user hit a bug in production and you need to see exactly what they saw — including the network calls that failed, the console errors that fired, and the DOM state at the moment things went wrong. Or when you need to know "how many users hit this funnel friction this week" without manually watching thousands of videos. Or when you want to auto-generate a Playwright test from a real user flow.
Use screen recording when…
You're recording yourself — narrating a Loom for an async standup, walking a customer through a feature, demoing in a sales call, or shipping a video doc for the team. Screen recording is for human-produced content; session replay is for machine-captured behaviour.
Why most teams end up using both
Session replay captures production user behaviour automatically. Screen recording captures human-produced explainers, demos, and walkthroughs. They're complementary — not substitutes — and the tooling overlap is near zero. Many teams use Loom for async communication and a session-replay tool (Relyv, Hotjar, FullStory) for production observability.