Alice Liddell was the girl who probably inspired the character of Alice in Wonderland and Through the Looking Glass. She met Carroll in 1856, when she was four-years-old, while he was photographing a cathedral. Carroll then became intimately involved with the child and her family. He would take the children out and tell them fantastical stories to pass the time.
There is a great deal of controversy surrounding their relationship, especially because he used the children as subjects for his photography, but their is no clear evidence to substantiate these claims. There was also gossip connecting him with the family governess and with Lorina, Alice's older sister.
It is actually uncertain whether Alice in the books is Alice Liddell. Carroll would claim later in his life that Alice is a completely fictional character. However, the Alices share the same birthday, and the acrostic poem at the end of Through the Looking Glass spells out "Alice Liddell."