In California, Burglary is defined as an entry into a building or structure with the intent to commit a theft or any felony while inside. Burglary is divided into two degrees. If the structure entered by the person intending to commit a theft or any felony is a dwelling, the crime is burglary in the first degree. All other burglaries are second degree.
A typical first degree burglary would be someone breaking into a residence and stealing something from inside. Of course, the intent to commit any other felony inside the residence would make the crime a burglary. For example, when a person enters a home intending to commit a murder or an assault inside, it's still a burglary even if nothing is stolen.
Second degree burglaries occur in a much wider variety of circumstances. Going into a store with the intent to shoplift is a burglary. Breaking into a car to steal something from inside is a burglary.
It is important to note that there is no requirement that entry be forced. The key is the intent of the person committing the crime: can the prosecution prove that he or she intended to commit a theft or any felony at the time he or she entered the structure? There is rarely direct evidence of intent. This element of the offense is usually met by circumstantial evidence. If the prosecutor can prove that the defendant did in fact enter a structure, and that the defendant committed a crime inside, the jury will be asked to infer from these facts that the defendant had intent to commit the crime at the time the defendant entered.
The intent must be formed prior to or at the time of entry. If the defendant enters the structure without intent to commit theft or any felony, but decides once he's inside that he wants to commit a theft or any felony, it's not a burglary.