Abishag was a Shunammite, Israelite woman who lived during the end of David's reign and was renowned for her beauty. Abishag was brought in to comfort and warm David in bed in his old age, though she remained a virgin. After David's death, his son, Adonijah tried to usurp the throne from Solomon by asking for her hand in marriage.
Abishag was a Hebrew Israelite woman born in the Tribe of Issachar. She lived in the village of Shunem during her early life. She was named "Ignorant Father", perhaps reflecting that she had a foolish father. She eventually became renowned for her beauty.
In David's oldest years he was not able to keep warm and comfortable in his bed. David's servants wished to keep their master warm so they decided to search all of Israel for a beautiful virgin to wait on the King. After searching the entire land of Israel, Abishag was brought in to be the King's Maiden.
This may have been a choice of hers or it may have been involuntary. Either way it would have been frowned upon for her to refuse. Abishag likely would have left the village of Shunem and taken to the Royal Palace in Jerusalem. As a maiden Abishag brought the King whatever he wished and did whatever he wanted. Most importantly she lied next to him at night in order to keep him warm (some would suggest this has to do not only with literal warmth, but in David's old age he may have had poor circulation leading to impotence). David's staff may have intended or expected David to sleep with her, but he did not.
Even when David was out of bed Abishag would be at his side attending to him. Abishag attended David at his throne- found evident by Abishag being present when Bathsheba came to protest Adonijah's kingship.
A short time later David died and Solomon became King. With her master dead, Abishag was now a servant of King Solomon. Solomon's brother and David's eldest son, Adonijah, noticed Abishag's beauty and wished to have her as a wife. Adonijah wanted the throne and having Abishag as his wife would also take power from his brother. Adonijah came to Bathsheba and asked to her to ask Solomon to give Abishag's hand in marriage to him. Adonijah reasoned that the Kingdom was his, but God's decree forfeited his right to King. Having Abishag as a wife would be pleasing to him and a method for him to gain power he felt he deserved.
Bathsheba went before Solomon and asked him to give Abishag's hand in marriage to Solomon. Hearing this Solomon was outraged, suspecting that Adonijah wished to usurp the throne from him. Adonijah was executed and some of his associates were also killed or exiled.