What is the relationship between the two terms? Is it that an ISR will be executed immediately, but an aperiodic task can be scheduled later (using the principle of a polling server or deferred server)?
They are completely different concepts.
Tasks are somewhat independent blocks of code, being executed by OS and managed by its task manager.
Interrupts are a way to signal to a task to temporarily stop it's execution. It can be SysTick interrupts from the OS that cause tasks to switch periodically to distribute time of processing between them. It could as well be user-initiated hardware interrupts by the input through peripherals.