In the Canon Law of the Catholic Church, the term latae sententiae refers to a penalty that is incurred automatically without the need for a Bishop or other Church superior explicitely declaring it.
The latae sententiae penalty is always connected to a specific offence, so if someone commits such an offense, he/she is automatically subjected to the penalty, providing the general conditions of being punishable are met (such as age, free will, etc).
Only those penalties are latae sententiae that Canon Law expressly declares as such. All others are ferendae sententiae and do not apply until a Church authority officially proclaims them.
An example is in Canon 1398, which states: "Qui abortum procurat, effectu secuto, in excommunicationem latae sententiae incurrit." That is, "A person who actually procures an abortion incurs a latae sententiae excommunication." The effectu secuto (actually) clause is very important as Canon Law requires that all laws about penalties are subject to strict interpretation. That means that if a nurse assists a doctor by handing him whatever tools he needed to perform an abortion, the nurse will not be excommunicated. The doctor, however, will be excommunicated just for performing the abortion even if no one outside the room ever finds out about it.