Apex can be executed synchronously or asynchronously.
Synchronous:
In a Synchronous call, the thread will wait until it completes its tasks before proceeding to next. In a Synchronous call, the code runs in single thread.
Example:
- Trigger
- Controller Extension
- Custom Controller
Asynchronous:
In a Asynchronous call, the thread will not wait until it completes its tasks before proceeding to next. Instead it proceeds to next leaving it run in separate thread. In a Asynchronous call, the code runs in multiple threads which helps to do many tasks as background jobs.
Example:
- Batch Apex
- @future Annotation
- Queueable Interface
- Scheduled Apex
