This sugya discusses the practical Halachic applications of a "Shomer" with regards to the prohibition of Yichud.  Specifically, we discuss the halachic distinctions regarding the age, gender, and number of minors present, and their relationship to the woman.  The differences of opinions are based upon the reasons that a minor would be considered a deterrent (i.e. a basic comprehension of what is happening around him without being fully and completely aware).

This has countless Halachic ramifications in our daily lives.  Some examples include:  servicemen/cleaning women working in a home when there is a child present (in addition to the woman/man), and girls babysitting for boys.