The number of active pills, and the level of hormones.
Junel Fe 24 and Junel Fe 1/20 contain the same level of hormones, so are a lower dose. But Junel Fe 24 is a 24-day pill, while Junel 1/20 is a 21-day pill. These are more suited to women who are sensitive to higher hormone levels, or who tried the higher dose and got side effects.
Junel Fe 1.5/30 is a higher-dose alternative.
All these packs come in 28-pill packs, with inactive iron pills as spacers for the seven- or four-day break.