Alcott employs the literary device of allusion, using John Bunyan's Pilgrim's Progress as the frame for her own novel. As in Bunyan's allegorical work, where the aptly named Christian embarks on a moral journey to spiritual maturity, the four March sisters also journey to maturity, becoming little women. To emphasize the connection to the earlier work, Alcott has Marmee give each of the girls a children's version of Pilgrim's Progress as a Christmas gift early in the novel and says she hopes they will take its lessons to heart. To underline that the moral lessons learned parallel Bunyan's book, chapters are named after the allegory, such as Vanity Fair ("Meg Goes to Vanity Fair") and the Valley of the Shadow of Death (Amy's Valley of Humiliation"). The lessons, however, are updated and made relevant to mid-nineteenth century girls: for example, in Alcott's version of Vanity Fair, Meg allows her rich friends to dress her up as a "fashion plate" for a dance, only to be humiliated when Laurie and others see her as a phony.
Little Women is also unabashedly a novel of sentiment. Sentiment, much beloved in the Victorian era, uses events and characters' reactions to them to elicit strong emotional responses from readers. Alcott works to raise reader feelings with dramatic plot devices, such as Amy burning Jo's manuscript only to almost drown when ice skating. Alcott helps us to feel Jo's pain, remorse, and forgiveness as she saves her beloved sister.
Characterization helps to sharply delineate each of the four sisters by giving them distinct traits that are different from each other: it is difficult to mistake the proper, well-mannered, and very pretty Meg from the tomboyish, disorderly writer Jo or the vain, artistic Amy from the shy homebody Beth.
Alcott also uses archetypal motifs to help us to easily identify situations in the novel: Laurie is, for example, the archetypal lonely little rich boy who needs a loving and cheerful home life while the March sisters are poor materially but rich in the intangible gifts of love, companionship, and merriment.
On a more granular level, as other answers have noted, Alcott uses such literary devices as similes, metaphors, imagery, and personification to build the vivid world of the March sisters.