What is the purpose of the Angel Tree charity?

There are actually two charities known as Angel Tree:

The Salvation Army's Angel Tree program makes sure every American child has gifts on Christmas morning. People in need of assistance sign up with the Salvation Army. Then, on Black Friday, Angel Tree tables are set up at malls. Someone goes to the table and "adopts" a child. The Salvation Army provides donors with each child's first name, age, sex, size, and Christmas wish. Donors fill a big bag with new clothing and toys for each child. Children who are not adopted are taken care of with Salvation Army funds (money dropped in kettles, collected in telethons, or donated during the year).

Angel Tree is a Christmas outreach program for the children of prisoners sponsored by the Prison Fellowship International.