People watching or crowd watching is the act of observing people and their interactions, usually without their knowledge. This differs from voyeurism

Voyeurism

In clinical psychology, voyeurism is the sexual interest in or practice of spying on people engaged in intimate behaviors, such as undressing, sexual activity, or other activity usually considered to be of a private nature....

 in that it does not relate to sex

Sex

In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...

 or sexual gratification. Eavesdropping

Eavesdropping

Eavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary...

 may accompany the activity, though is not required.

Though often a casual hobby, it can be used formally as a means for sociological, anthropological or psychological research. Naturalistic observation

Naturalistic observation

Naturalistic observation is a research tool in which a subject is observed in its natural habitat without any manipulation by the observer. During naturalistic observation researchers take great care to avoid interfering with the behavior they are observing by using unobtrusive methods...

is a more formal way of describing people watching in an academic sense.