Lambs are young Sheep, peaceful creatures that are among those loyal to Aslan in the world of Narnia.
At the end of The Voyage of the Dawn Treader, Aslan transformed himself into a lamb, and morphed back when Lucy, Edmund, Eustace, and Reepicheep arrived in Aslan's Country.
In The Last Battle, one unnamed lamb asked Shift the Ape why Aslan would send them to work in Calormen, whose people worship Tash. This is where the reader is introduced to the lie of Tashlan, where Aslan and Tash are believed to be the one and the same. The lamb later joined the other animals that entered Aslan's Country.