Breaking and entering are two of the elements necessary to constitute a burglary, and are generally defined in the law dictionary as "the gaining of unauthorized, illegal access to another's premises, as by forcing a lock." Another entry has it as: "trespassing for an unlawful purpose; illegal entrance into premises with criminal intent". The simple act of pushing open a door to a premises which has been left ajar is enough to constitute breaking and entering, being that you have no permission to enter, and are then trespassing.
Breaking and entering, in and of itself, is a misdemeanor crime when unaccompanied by more serious offenses. Broken down individually, breaking does not require that anything be "broken" in terms of physical damage occurring. It can be either actual, such as by forcing open a door, or constructive, such as by fraud or threats. Entering can involve either physical entry by a person or the insertion of an instrument with which to remove property. Insertion of a tool to gain entry may not constitute entering by itself. It is generally required at common law that entry occurs as a consequence of the breaking. For instance, if a malfeasor opened a window partially by using a pry bar and then noticed an open door through which he subsequently entered the building, there is no burglary per se at common law.
Breaking and entering can have serious consequences if the perpetrator, whose intent may be innocent in some cases, e.g. just to see if a neighbor, acquaintance, pet, etc. has entered onto said property, goes unawares of property owners' rights to defend themselves and their property, or unawares of the punishment involved. Surely in most cases "B & E" is a precursor to another, more serious crime, such as burglary, larceny, arson, kidnapping, rape, assault, battery, murder, etc., and thus is taken very seriously.