Putting it in very simple terms, asynchronous sequential circuits do not have a master clock that synchronizes all the components in the circuits. Hence, the name 'asynchronous'. Synchronous circuits, on the other hand, do have a master clock that synchronizes the components.
Asynchronous sequential circuits are just combinational circuits with feedback. The feedback path in Synchronous sequential circuits consists of memory elements like flipflops.
If you want to understand all these concepts in depth, I'd suggest you to go through Digital Design by Morris Mano. It's all explained very beautifully in the book.
Hope you found it useful