In a nutshell, asynchronous Apex is used to run processes in a separate thread, at a later time. An asynchronous process is a process or function that executes a task "in the background" without the user having to wait for the task to finish.
You’ll typically use Asynchronous Apex for callouts to external systems, operations that require higher limits, and code that needs to run at a certain time. The key benefits of asynchronous processing include:
User efficiency: Let's say you have a process that makes many calculations on a custom object whenever an Opportunity is created. The time needed to execute these calculations could range from a minor annoyance to a productivity blocker for the user. Since these calculations don't affect what the user is currently doing, making them wait for a long running process is not an efficient use of their time. With asynchronous processing the user can get on with their work, the processing can be done in the background and the user can see the results at their convenience.
Scalability: By allowing some features of the platform to execute when resources become available at some point in the future, resources can be managed and scaled quickly. This allows the platform to handle more jobs using parallel processing.
Higher Limits: Asynchronous processes are started in a new thread, with higher governor and execution limits. And to be honest, doesn’t everyone want higher governor and execution limits?
Asynchronous Apex comes in a number of different flavors. We’ll get into more detail for each one shortly, but here’s a high level overview.