First of all let me ask, do you know what a caricature picture is before you even start to try to draw caricature pictures?
The critical difference between the caricature drawing and a portrait of a person is the deliberate distortion of the subject's categorizing features. This is tricky because you distort the features from which to identify the subject and if you fool around with the features too much, you run the risk of losing the similarity with the person. You need to strike a balance between the distortion and the similarity of features keeping some of the natural look to maximize or accentuate the features that distinguish the person as them. If you lean too much against the natural flow of the face of a person you lose similarity, draw a cartoon portrait of the Rolling Stone, Mick Jagger, with thin lips and a small mouth or Beatle, John Lennon, without the distinguishable hooked nose and round glasses, and you will be creating something that is unrecognizable ?
A few examples of how the dictionary would define caricatures:
* A representation, especially visual and or literary where the subject has special characteristics, characteristics that are intentionally exaggerated to make them look comical or grotesque.
* The skill of the creation of such comic representations in picture or description format that is ludicrously exaggerating peculiarities or defects in a person.
* Grotesque imitations or exaggerations, the art or process of producing such pictures, descriptions, etc.
Being able to draw portraits of people, having the knowledge, understanding and feeling for drawing faces is very helpful, enabling you to be more able to draw effective caricatures of them. As for the difference between cartoons and caricatures, it does not exist, they are really the same as there is no difference as far as I can tell. Cartoons are simply caricatures of a person or something you are able to recognize explicitly as someone or something familiar. If people can see someone they recognize in a cartoon drawing the viewer would categorize it as a caricature. Only to get confused about the whole thing a little more, caricatures may not always be drawings they could also be paintings, sculptures, or maybe even photographic caricatures.
You could draw a picture as a portrait but if you then drew the proportions wrong and it was still recognizable as the person you intended, it should really be seen as a caricature rather than a portrait. If the artist's intentions are to draw caricature pictures then they do not have to be in the cartoon format they can be in any format and still be caricatures but most of them are drawn as cartoons.