JPEG
The dominant lossy image format, using frequency-domain compression to achieve small files at the cost of some quality.
Measured in
quality 1-100
In depth
JPEG discards information the eye is least likely to notice, particularly fine colour detail, and stores eight bits per channel. Every save re-compresses and degrades the file further, which is why it should be an export format rather than a working one. Its universality is unmatched — nothing else opens everywhere without question.
Examples in practice
A quality-90 JPEG is visually indistinguishable from the original at a quarter of the file size.