Shoulder surfing is the practice of watching a person who is getting money from a machine, filling out a form, or doing something similar, in order to find out their personal information.
The Oxford Advanced Learner's Dictionary and the Oxford Advanced American Dictionary give the same definition and both label the term informal. Their examples show what is at stake: "The PIN numbers were obtained by shoulder surfing," and the advice to "learn how to spot shoulder surfing." The information at risk is whatever a person enters at a machine or writes on a form while someone else can watch.
Shoulder surfing works by sight, which separates it from eavesdropping. Oxford defines to eavesdrop as to listen secretly to what other people are saying, and traces it to an early seventeenth-century back-formation from eavesdropper, "a person who listens from under the eaves." The eavesdropper collects what is said; the shoulder surfer collects what is shown, typed or written.