Short answers, pulled from the story.
As You Like It is believed to have been written in 1599, with external and internal evidence placing composition between the end of 1598 and the middle of 1599. It was first published in the First Folio in 1623; no Quarto edition exists.
The direct source is Thomas Lodge Jr's prose romance Rosalynde, Euphues Golden Legacie, written in 1586-87 and first published in 1590. Lodge himself based his story on an older work known as "The Tale of Gamelyn."
Jaques is a melancholy traveler attached to the exiled Duke Senior's court in the Forest of Arden. His speech beginning "All the world's a stage" appears in Act II, Scene VII, and describes human life as a play in seven ages, from infancy to a final "second childishness and mere oblivion, sans teeth, sans eyes, sans taste, sans everything."
Duke Frederick banishes Rosalind from court, so she flees with her cousin Celia to the Forest of Arden. To travel safely, Rosalind disguises herself as a young man named Ganymede, while Celia takes the name Aliena. In the forest, Rosalind uses the disguise to test and guide Orlando's love without revealing her identity.
The Forest of Arden is a deliberately layered setting. The Arden edition of Shakespeare suggests the name combines Arcadia and Eden. It also evokes the real English Forest of Arden, the ancestral home of Shakespeare's mother Mary Arden, who was born at Wilmcote near Stratford-upon-Avon. The Oxford edition treats it as an anglicisation of the Ardennes region of Belgium, Luxembourg, and France.
Shakespeare wrote roughly 55 percent of As You Like It in prose, which was unusual for a play with aristocratic central characters. He deliberately reversed the convention of the time: Rosalind, a duke's daughter, speaks in prose, while the shepherd Silvius speaks in verse. George Bernard Shaw described the prose as "brief and sure."