当一个经理有几个员工，并且可以给他们每个人一个单独的任务时，并行性就会发生。工人们做他们的工作，并向经理提供结果。如果任务不能完全分离，例如任务之间的结果有一定的依赖性，或者需要在没有其他推理的情况下使用相同的资源，那么并行度就受到这些约束，不能完全实现。

Concurrency happens when a manager has several tasks but only less workers, hence some workers are given more than one task. Any worker given multiple tasks, divides each original given task to several steps and does the steps interleaved, each task result will be given back to manager as soon as every steps of it finished. Manager receive a task result while other tasks started and progressed several steps but have not finished yet. If any worker with multiple task decides not to start a single step of a given task before finishing every steps of an already started task, this is called sequentiality.

Asynchrony is any of the two above mixed or separated, seen from the manager's point of view. When the manager assigns the tasks to either few or enough workers he shall not be awaited stalled until any results are given back. He can do his personal jobs or whatever, while jobs are progressing. Usually workers do not decide how tasks should be divided into steps. An inversion of control means manager decides over steps and gives single steps to workers. So when he receives a step result from a worker, give him another step, maybe from another task. The whom under control is responsible for composing back step results into task results as well. So Asynchronicity comes with responsibility for control and probably coordination. If any worker is urged to work sequentially, from manager's point of view he is a synchronous worker.

Summary As it's simple to guess, full parallelism is an unrealisable idea unless otherwise in rare mostly trivial cases. Since reality comes with interdependent tasks and shared resources and lack of workers. So concurrency is the reality. From manager's point of view this concurrency is best if it does not hinder him from fine controlling the tasks, and if positive it is called asynchronous. Also computer software engineering best practices, augmented by S in SOLID principle, historically made servers single step runners called micro-services, this returned back control to the clients. So current situation is concurrency from server point of view and asynchronicity from client point of view.

并发性可能发生在以下几种场景中:

异步——这意味着你的程序执行非阻塞操作。例如，它可以通过HTTP发起对远程资源的请求，然后在等待接收响应的同时继续执行一些其他任务。这有点像你发了一封电子邮件，然后继续你的生活，没有等待回复。

并行性——这意味着您的程序利用多核机器的硬件，通过将工作分解为任务，在同一时间执行任务，每个任务都在单独的核心上执行。这有点像洗澡时唱歌:你实际上是在同时做两件事。

多线程——这是一种允许不同线程并发执行的软件实现。多线程程序似乎同时在做几件事，即使它运行在单核机器上。这有点像通过各种IM窗口与不同的人聊天;虽然你实际上是在来回切换，但最终结果是你同时进行了多个对话。

并发+并行都意味着同时运行多个任务。我看到一些人认为这是有区别的，但这取决于你咨询的参考文献，没有真正的正确或错误的答案。

Asynchronous: In some communities this means non-blocking code, which can mean two things: It almost always means it will not block an OS thread. However, non-blocking can optionally mean that the next line of source code in a function will continue to run without delay. In Python await asyncio.sleep(5) blocks execution of the function, but not the OS thread, and that is considered async. In Golang, you have "goroutines" that similarly to Python's await, they block execution of code, but not OS threads, however, this is not referred to as async in the Golang community. It's just concurrent programming.

并发和并行实际上是相同的原理，两者都与同时执行的任务有关，尽管我想说并行任务应该是真正的多任务，“同时”执行，而并发可能意味着任务共享执行线程，同时仍然看起来是并行执行。

异步方法与前两个概念没有直接关系，异步被用来呈现并发或并行任务的印象，但实际上异步方法调用通常用于需要在当前应用程序之外执行工作的进程，我们不希望等待和阻塞应用程序等待响应。

例如，从数据库获取数据可能需要时间，但我们不想阻塞UI等待数据。异步调用接受回调引用，并在请求被放置到远程系统后立即将执行返回给您的代码。当远程系统执行所需的任何处理时，UI可以继续响应用户，一旦它将数据返回给回调方法，那么该方法就可以适当地更新UI(或移交更新)。

从用户的角度来看，它看起来像多任务处理，但它可能不是。

EDIT

可能值得补充的是，在许多实现中，异步方法调用会导致线程启动，但这不是必要的，这实际上取决于正在执行的操作以及如何将响应通知回系统。

总结

并发(Concurrent):多件事情似乎同时发生(具有在并发任务之间快速切换的能力;它们是并发的，因为每个任务都想要一块资源，CPU等等。)

并行是指同时发生多件事情(执行线程的数量与执行内核的数量密切相关)

异步是简单的非阻塞，当我们必须等待的事情不会让我们忙于等待(需要某种通知机制从我们离开的地方继续)

并发性意味着同时执行多个任务，但不一定是同时执行。当你必须执行多个任务，但你只有一个资源时，我们就会追求并发性。在单核环境中，并发是通过上下文切换实现的。

并行性就像同时执行多个任务，就像你可以一边唱歌一边洗澡。现在你在并行地做这些任务。

术语异步与线程执行有关。在异步模型中，当执行一个任务时，您可以切换到另一个任务，而无需等待前一个任务完成。

异步编程帮助我们实现并发性。多线程环境中的异步编程是实现并行的一种方法。