Efficient or effective?

What’s the difference between efficient and effective? When should I use what? Are they synonyms?

Lately I have heard effective used instead of efficient. I use effective only as in “the cease-fire was effective immediately“.

I hope somebody can explain how they use the words. 
According to my dictionary there is no (or very small) difference between them.

A question about #1
What do you prefer in it: some, somebody, someone or pehaps anybody (anyone)?

#2: Someone.

I would say efficient is a way to describe achieving maximum productivity/ getting the most out of something. A good synonym for this could be productive or well-organised.

Whereas effective describes being successful or achieving an intended result. A synonym for this could be potent, successful or functional. 

Great clarification Leia! Couldn’t have said it better

Aha, that has been hard for me, as a non-English native, to understand. Thanks!