What is concurrency and parallelism? How do they differ? Can they coexist?

Concurrency is the ability of a system to manage multiple tasks at the same time by switching between them to efficiently utilize resources. Parallelism is the simultaneous execution of multiple tasks on different processor cores.

The main difference:

• Concurrency is about organizing work so that tasks can be executed sequentially but appear to be simultaneous.
• Parallelism is about executing multiple tasks at the same time in real-time.

They can coexist: a program can be concurrent (managing many tasks), and with multiple cores, these tasks can be executed in parallel.

In Go, for example, goroutines provide concurrency, and the scheduler can distribute them across multiple cores for parallelism.