Puccini's partnership with the playwright/librettists Luigi Illica and Giuseppe Giacosa was one of the most successful in the whole history of Italian opera -- a meeting of great artistic minds akin to Verdi's association with Boito and Bellini's with Romani. Although Illica and Giacosa are best remembered for their work with Puccini, each had an active career of his own.
Luigi Illica (1857-1919)
Luigi Illica had a rough beginning: at an early age he ran away to sea and in 1876 he found himself fighting the Turks. Three years later, however, he moved to the relatively peaceful enclave of Milan Italy and there began his literary career. In 1882 he produced a collection of prose sketches, Farfalle, effetti di luce, and the following year wrote his first play, I Narbonnier-Latour. His greatest success in this field of playwriting was a comedy in Milanese dialect, L'eriditaa di Felis (1891).
He began writing librettos in 1889. While his work on three of Puccini's operas is recognized as his chief contribution to the field, he also wrote librettos for several other composers, including those for Giordano's Andrea Chenier (1896--the same year as La Bohème), an opera still popular and performed today, and two operas of Mascagni.
Giuseppe Giacosa (1847-1906)
Giuseppe Giacosa began his professional life, not as a writer, but as a lawyer. He graduated in law from Turin University and immediately joined his father's firm in Milan. He moved permanently into the literary world, however, when his one-act verse comedy, Una partita a scacchi, became a popular success. From 1888-1894 Giacosa held the chair of literature and dramatic art at the Milan Conservatory.
The Puccini/Illica/Giacosa partnership was organized by the publisher Giulio Ricordi in 1893. The head of the most powerful publishing firm in Italy during the 19th century, Ricordi had the ability to make or break any young composer who came along, much in the same way that a CEO of a major record label can do today.
Having taken Puccini under his wing, Ricordi was intent on hiring the best writers to work with the young composer on his La Bohème -- he found them in Illica and Giacosa. The three had a very clear division of responsibilities when working together: it was Illica's job to plan the scenario (i.e. the opera's plan, and division into acts and scenes) and to draft the dialogue; next, Giacosa transformed the prose into polished verse; finally Puccini set this verse to music. This collaboration was such a success that the three worked together (dividing the responsibilities in the same way) on two other operas: Tosca (1900) and Madama Butterfly (1904).
The collaboration ended with the death of Giacosa in 1906. Puccini continued to discuss the idea of translating the story of Marie Antoinette into an operatic setting with Illica, but this project never came to fruition. For his final operas, Puccini turned to other librettists.
Sources: Julian Budden, "Luigi Illica" and "Giuseppe Giacosa" in The New Grove Dictionary of Music and Musicians.