Stands for "Interrupt Service Routine." An ISR (also called an interrupt handler) is a software process invoked by an interrupt request from a hardware device. It handles the request and sends it to the CPU, interrupting the active process. When the ISR is complete, the process is resumed.
A basic example of an ISR is a routine that handles keyboard events, such as pressing or releasing a key. Each time a key is pressed, the the ISR processes the input. For example, if you press and hold the right arrow key in a text file, the ISR will signal to the CPU that the right arrow key is depressed. The CPU sends this information to the active word processor or text editing program, which will move the cursor to the right. When you let go of the key, the ISR handles the "key up" event. This interrupts the previous "key down" state, which signals to the program to stop moving the cursor.
Many types of hardware devices, including internal components and external peripherals can sent interrupts to the CPU. Examples include keyboards, mice, sound cards, and hard drives. A device driver enables communication between each of these devices and the CPU. ISRs prioritize interrupt requests based on the IRQ setting of the device (or port). Typically the keyboard is at the top of the IRQ list, while devices like hard drives are further down.