The Dictionary of Classical Mythology explains that Zeus was enamoured of Europa and decided to seduce or rape her, the two being near-equivalent in Greek myth. He transformed himself into a tame white bull and mixed in with her father’s herds. While Europa and her helpers were gathering flowers, she saw the bull, caressed his flanks, and eventually got onto his back. Zeus took that opportunity and ran to the sea and swam, with her on his back, to the island of Crete.
In case people wonder about the bull.