What is the difference between implicature and entailment?

Question

When talking about pragmatics what is the difference between implicature and entailment?

^{PS. The book I was reading was Pragmatics by George Yule}

Answer 1

An entailment is a necessary implication: an inference from an utterance which must be true if the utterance is true.

An implicature is a cancellable implication: an inference from an utterance which we take the utterance to imply on its face, by 'default', but which may in the context of other information nonetheless not be true even if the utterance is true.

Take for instance the utterance

John's been in Paris for three years.

This utterance entails the inference that John was in Paris at some point in the past: if John has never been in Paris, then the utterance itself is false.

The utterance implicates the inference that John is still in Paris at the time of utterance: under ordinary circumstances an ordinary English hearer will understand this to be a continuative perfect. But that inference is not necessarily true; under some circumstances it may be that the utterance should be understood as an existential or resultative perfect:

John's been in Paris for three years. He's also been in London for five years, and in Vienna for two.
John's been in Paris for three years, if you need some pointers on how they do business there.