Katherine Howard was a younger daughter of Edmund Howard and his wife, Jocasta Culpeper. Her date of birth is not known for certain; the year of her birth is thought to be between 1521-1523. Jocasta already had several children by her first husband when she married Edmund. Edmund Howard was a younger son of the 2nd Duke of Norfolk and his first wife. Although from a powerful family, he had no lands or inheritance and therefore not much money. Edmund’s sister Elizabeth was the mother of Anne Boleyn, making Anne and Katherine first cousins. Jocasta died when Katherine was very young and Edmund re-married. His second wife was a widow named Dorothy with 8 children of her own. In 1531 Edmund was given a job as Controller of Calais.
While she was still a young girl, Katherine was sent to live at Lambeth Palace on the bank of the River Thames. It was the household of her step-grandmother Agnes Tilney; widow of the second Duke of Norfolk. It was here Katherine received her education. She was literate in English, played the lute, a good dancer and had good courtly manners. The Dowager Duchess’ household lacked discipline, however, with the young men and women doing what they pleased a lot of the time. The young men were able to sneak into the girls’ bedrooms at night.
Katherine soon started an affair with her music master, Henry Manox. He was employed to teach at the Dowager Duchess’ household in 1536. Katherine and Henry became close, although they never had sex. The Dowager Duchess caught the two of them together several times. The relationship between Henry and Katherine ended, and soon after Katherine was involved with Francis Dereham. 
Miniature portrait of Katherine Howard, painted by Hans Holbein the Younger
Francis Dereham was a gentleman in the Dowager Duchess’ household. Katherine was much more taken with Francis than she was with Henry; Francis would bring her presents and she secretly allowed him to come into the girl’s bedroom at night. Their relationship was sexual. They seemed to be in love with eachother, and agreed to marry when Francis returned from a trip to Ireland.
Katherine was chosen to go to the Royal Court in 1539 as a lady in waiting to the new Queen, Anne of Cleves. King Henry VIII was not happy with his new wife. Katherine soon caught his eye. King Henry started sending Katherine gifts in spring 1540, which she accepted happily. Her uncle, the Duke of Norfolk, encouraged the relationship. The Dowager Duchess was also encouraging, sending Katherine new clothes and giving her advice about what to do with the King. King Henry’s marriage to Anne of Cleves was annulled on July 9th 1540, Katherine married Henry on July 28th.
The King was very much in love with his new wife. She was granted most of the recently executed Thomas Cromwell’s estates, as well as two dissolved abbeys in Reading and Gloucester. Henry called Katherine his “rose without a thorn” and she took the motto “no other will than his”. He showered Katherine with gifts and jewels and gave her a lot of his attention.
Katherine was no older than 19 when she married the King. King Henry was 49, overweight and sick with an infected ulcer on his leg. In the spring of 1541, Katherine started a flirtation with a young member of the King’s privy chamber, Thomas Culpeper. They had flirted when she first arrived at Court, but as she had caught the King’s eye nothing came of it. The flirtation soon became an affair. Katherine wrote Thomas love letters and was infatuated if not in love. (http://englishhistory.net/tudor/letter13.html)
The secret meetings between Katherine and Thomas Culpeper continued while the Court went on progress to the North in 1541. One of Katherine’s ladies, Jane Boleyn, found out about the affair. Rather than tell anyone about it, she assisted Katherine and Thomas with their meetings and kept the affair a secret. If Katherine’s other ladies knew of the affair, they did not say anything. Francis Dereham returned to England during the royal progress and became Katherine’s private secretary. She also gave jobs to two women she had grown up with in the Dowager Duchess’ household; Katherine Tilney and Joan Bulmer became her ladies in waiting.
Mary Hall was another woman who had grown up in the household of the Dowager Duchess. She told her brother John Lascelles, who told Archbishop Cranmer, of Katherine’s affairs with Henry Manox and Francis Dereham. Mary also told of a pre-contract of marriage between Francis and Katherine, which would make her marriage to the King null and void. When King Henry discovered the rumours, he did not believe a word. He ordered a secret investigation into the rumours with the intention of clearing his wife’s name.
On November 5th 1541, Katherine was instructed to stay in her rooms at Hampton Court while the investigation took place. She did not know at first why she had been confined. On November 7th, she was questioned by Archbishop Cranmer. She denied there was a pre-contract of marriage between her and Francis, but eventually confessed to having a sexual relationship with him. She confessed her affair with Henry Manox too, but denied having sex with him. 
Katherine Howard at Syon Abbey, played by Tamsin Merchant in the Showtime TV series “The Tudors”
Both Francis Dereham and Henry Manox were arrested on November 5th and questioned by Thomas Wriothesley. Henry Manox confessed to having his affair with Katherine, and confirmed they had not consummated it. Francis Dereham confessed to having carnal knowledge of the Queen, although not after her marriage, only before. Ladies from Katherine’s household were also questioned about the affairs and the Dowager Duchess was arrested.
On November 11th, under further questioning, Francis revealed the Queen’s current affair with Thomas Culpeper; he was arrested the next day and questioned. Thomas confessed to a sexual relationship with Katherine, but later changed his story and said the affair was not sexual. His belongings were searched. Jane Boleyn’s knowledge of the affair was discovered when a letter Katherine had written to Thomas, naming her, was found. She too was arrested and sent to the Tower.
On November 13th 1541 the Queen’s household was closed and on the 23rd she was stripped of her title of Queen. She was to be known as the Lady Katherine Howard and was sent to Syon Abbey. On December 1st, Thomas and Francis went on trial. They were found guilty and executed on December 10th. The Dowager Duchess was charged with concealing the affairs and kept in prison; she was later released and pardoned by the King. Henry Manox was also released.
Katherine remained at Syon Abbey. An act of attainder to convict both herself and Jane Boleyn of treason was put to Parliament on January 21st 1542. The bill passed on February 7th: it was now an act of high treason to keep any past sexual relationships a secret from the King, if you were to become his wife. Katherine was moved from Syon Abbey to the Tower of London on February 10th. She passed both Thomas’ and Francis’ heads on the way; they were displayed on pikes at London Bridge. Katherine’s execution was scheduled for February 13th. She was to be beheaded on the same scaffold as Jane Boleyn. On the evening of the 12th of February, she asked for the block to be brought to her, so she could practice how to lay her head. This request was granted.
Katherine was beheaded with a single stroke of the axe a little after 7am on February 13th, 1542. She did not declare “I would rather die the wife of Culpeper!” as is often said. She made a good speech, begging pardon for her sins and praying for the King’s preservation. An eyewitness, Ottwell Johnson, said of her;
“I saw the Queen, then Lady Rochford, suffer within the Tower. Whose souls be with God, for they made the most Godly and Christian end.”
Katherine Howard at her execution, played by Tamsin Merchant in the Showtime TV series “The Tudors”
Further reading-
“The Six Wives of Henry VIII” by David Starkey
“Tudor Queens of England” by David Loades