Overview
IRQs are the primary way hardware devices get the CPU's attention. For example, when you press a key, the keyboard controller sends an IRQ to the CPU.
IRQ Lines
Historically, PCs had a limited number of physical IRQ lines (0-15). Modern systems use Message Signaled Interrupts (MSI), which send interrupts as data packets over the PCIe bus, effectively removing the limit on the number of interrupts.
Priority
Interrupts are prioritized so that critical events (like a timer or hardware failure) are handled before less urgent ones (like a mouse movement).