Interrupts are very high-priority signals that halt the processor's activity and prepare the processor for a new activity, if only very briefly. Interrupts consume processor time and disrupt the processor's work, but they are essential to a preemptive multitasking system.
Client connections involve many interrupts. The network adapter card generates an interrupt when it receives a new packet or completes a transmission. The system collects some very basic information and then adds one or more deferred procedure calls (DPCs) to the queue to handle the next steps in the process.
DPCs are similar to interrupts except that they have a lower Interrupt Request Level (IRQL). (IRQL is the priority scale used for objects like interrupts. It is different from the priority scale that the microkernel uses to schedule processes and threads.) Unlike interrupts, DPCs can be delayed, allowing the processor to complete higher priority work. When the DPC gets processor time, the work of establishing the connection can proceed.