Short answers, pulled from the story.
Lady Jane Grey was an English noblewoman proclaimed Queen of England and Ireland on the 10th of July 1553. She was deposed nine days later when the Privy Council switched its allegiance to her cousin, Mary I, and she was beheaded for high treason on the 12th of February 1554.
Lady Jane Grey lost the throne because support for her rival claimant, Mary I, grew rapidly after Edward VI's death, and the Privy Council abandoned Jane to proclaim Mary queen on the 19th of July 1553. Most of Jane's original supporters, including the council that had signed her into power, switched sides within days.
Lady Jane Grey was the great-granddaughter of Henry VII through his youngest surviving daughter, Mary, and the grandniece of Henry VIII. She was first cousin once removed to Edward VI, Mary I, and Elizabeth I, and Henry VIII's will placed her in line to the throne after his own children.
Edward VI named Lady Jane Grey his successor because his Catholic half-sister Mary was next in line and he wanted a Protestant heir who would preserve the reformed Church of England. His will, drafted as 'My devise for the Succession' and issued as letters patent on the 21st of June 1553, disinherited both Mary and Elizabeth and named Jane and her male heirs instead.
Lady Jane Grey married Lord Guildford Dudley, a younger son of John Dudley, 1st Duke of Northumberland, on the 25th of May 1553. The wedding took place at Durham House as part of a triple ceremony that also married two of the couple's siblings.
Lady Jane Grey was beheaded at the Tower of London on the 12th of February 1554, shortly after her husband Guildford was executed the same morning. She had been convicted of high treason in November 1553, and her suspended death sentence was carried out after her father joined Wyatt's Rebellion against Queen Mary in January 1554.