Born in Spain in 1488, Joseph Karo moved to Turkey with his family. He became a brilliant Jewish scholar, but, at the same time, became involved in Kabbalah, Jewish mysticism. He met and was strongly influenced by Solomon Alkabetz. Together, they created the ritual of Tikkun Leil Shavuot, the tradition of staying up all night on Shavuot evening. It was during one of these all-night rituals, that Karo was visited for the first time by his maggid, an angel who perched on his shoulder and kissed Jewish law into his mouth.
Inspired by this combination of mysticism and Halachah, Joseph Karo began writing his major work, the Beit Yosef in 1522. It took twenty years to complete. The Beit Yosef, written as a voluminous commentary to an earlier code, the Tur, was Joseph Karo's attempt to codify all of Jewish law. It was a huge undertaking because Karo tried to show the origins of all Halachic decisions in his code before arriving at each decisive ruling. He applied Talmud, Alfasi, RaMBaM's Mishneh Torah, Adret's responsa and a legion of other post-Tur legal decisions. He showed familiarity with all of the great legalists and included varying community customs.
Karo then wrote a short digest of all of the laws he had dealt with so extensively in the Beit Yosef. Aimed as a simple guide for young students, this digest listed in concise Hebrew what a Jew was supposed to do in each circumstance of life. Karo tried to follow a formula for declaring what the Halachah was when previous sages were in disagreement, but, like RaMBaM, he created controversy by his decisions.
Karo assumed that the scholarly reader who wanted the ocean of legal discussions would refer to the Beit Yosef. He therefore wrote the Shulchan Aruch in the same format as his more scholarly work. He divided it into easily-found sections and paragraphs. It's clear that he intended for his two works to be read together. Being in the right place at the right time foiled his plan.
The Shulchan Aruch, the simple compendium of stated rules, became tremendously influential in the Jewish world because it was the first code to be printed on the revolutionary new invention, the printing press. Joseph Karo had moved to Tzfat, the city of Jewish mysticism in Israel. Tzfat had a printing press, and the Shulchan Aruch was one of its earliest ventures. It was printed in 1565 and distributed around the Jewish world. This meant that many more printed copies were available of the Shulchan Aruch than any other code written in manuscript form.
However, because of Joseph Karo's Sephardic background, the Shulchan Aruch did not include Ashkenazic and Polish customs. Therefore, although the text was available to them, Ashkenazic Jews were unwilling to accept the code as authoritative.
Moses Isserles, a great Polish rabbi, added a commentary of Ashkenazic customs to the Shulchan Aruch. First published in 1569, Isserles' Mappah, Tablecloth, was responsible for the Shulchan Aruch, the Prepared Table, being accepted as a major legal work.
It took several more commentaries and editions before the Shulchan Aruch became the code of Jewish law. However, traditional Jews today view Joseph Karo's Shulchan Aruch along with Isserles' Mappah as the accepted authoritative starting points for Jewish law. The process of expounding and amplifying halachah through modern responsa and papers continues to this day. However, one event concretized the Shulchan Aruch for the 17th century Jewish world.
While in Tzfat, Karo became embroiled in Berav's Semichah controversy. He was one of the rabbis who was ordained.
Although it has very little to do with Jewish history, it should be noted that Joseph Karo impregnated his (fifth) wife when he was 81 years old.
Sources: Gates to Jewish Heritage