The difference between continual and continuous can be hard to notice.

The Oxford English Dictionary defines continual and continuous as follows:

Continual:      Forming a sequence in which the same action or event is repeated frequently.

ContinuousForming an unbroken whole; without interruption.

Interruption of time is key to understanding the difference between these two words.

And one other key difference between the two is that continuous can be used to refer to both to space as well as time.

Incorrect:         His continuous verbal attacks sealed the fate of the coalition talks between the two parties.

Correct:             His continual verbal attacks sealed the fate of the coalition talks between the two parties.

Incorrect:         This role requires continuous vigilance.

Correct:            This role requires continual vigilance.

You can further explore this topic on writing well here.