California Penal Code Section 459 defines burglary as the act of entering a building or structure with the intent to commit theft or any felony inside. This law outlines specific circumstances under which an individual can be charged with burglary, emphasizing the unlawful entry and the intent to commit a crime once inside. Understanding this section is crucial for recognizing the different types of burglary, potential penalties, and how intent plays a vital role in proving the crime.