The British spellings are with the double l and the American ones with single l.

cancelled — canceled
marvellous — marvelous
traveller — traveler

American English doesn’t usually double up the l but sometimes it does if the stress falls on a syllable other than the first, which we'll see later.

As I've said before, it's always great to know the difference between British and American spellings because it stops you getting confused, because of spellcheck internet, books, etc. 

So let's look at why and when we double up the final l.

Usually in British English, we double up the final l when a word is more than one syllable and we add a vowel suffix - ing, -ation, -ed, -ous, -er) But like all rules there are exceptions, which we'll look at later, and as I said we sometimes double up in American English too so watch out for those later.

cancel has two syllables and when we add a vowel suffix endings, we double up the l:
British: cancel — cancelling, cancelled, cancellation
American: cancel — canceling, canceled, cancelation

British: marvel — marvellous, marvelled, marvelling
American: marvel — marvelous, marveled, marveling 

British: travel — travelling, travelled, traveller
American: travel — traveling, traveled, traveler

Brit: model — modelled, modelling, modeller
Am: model — modeled, modeling, modeler

Brit: fuel — fuelled, fuelling
Am: fuel — fueled, fueling

Brit: initial — initialled, initialling
Am: initial — initialed, initialing

American usage agrees with British on words that have the stress on the second syllable:
control ("con TROL") — controlled, controlling
patrol — patrolled, patrolling
expel — expelled, expelling
extol — extolled extolling
annul — annulled, annulling

American and British agree not to double the end l in parallel — paralleling, paralleled 

We don't double the l in these words:
appeal — appealing, appealed
devil — devilish
loyal — loyalist
travel — travelogue

We have some root words that are spelt differently in British and American

enrol vs. enroll

British = enrol vs. American = enroll
British English: enrol — enrolling, enrolled, enrolment (only double up with vowel suffixes but American enrollment)
American English: enroll is spelled with double l so just add the suffix ending: enrolling, enrolled, enrollment

Careful:

install: installing, installed but instalment (British — drop the l) and installment (American — keep the l)

fulfil (Brit): fulfilled, fulfilling, fulfiller but fulfilment
fulfill (Am): fulfilled, fulfilling, fulfiller and fulfillment 

Check out the Adding all, till, fill, full lesson