Define an illustrator in layman’s terms.

An illustrator is a person (visual artist) who creates art pieces.

Why is an illustrator often mistaken for a graphic designer?

This is usually because an illustrator's work can be used in design work too. They can create art pieces to be used in branding, product designing, books and so on.

What are key differences between and illustrator and a graphic designer?

A graphic designer creates design elements such as fonts, and other brand identity elements while an illustrator will work on things such as comics, book houses, product packaging, illustrations and anything that involves sketching and drawing.

A graphic designer concentrates more on brand design, product design, website design, and publication design whereas an illustrator focuses more on drawing and painting.

A graphic designer has the ability to organise brand campaign materials such as advertisng material while an illustrator will only support in case they need an artistic impression.

An illustrator aims to produce artwork for artistic enjoyment.

An illustrator creates an image that represents a subject addressed. He makes it easier for the reader to form an idea that the text is describing while a graphic designer is presenting content. He is making content acceptable, treatable and facilitating navigation of any content.

Is it advisable for one to pursue only one and not both?

One is at a very great advantage if they can do both so it's advisable not to pursue only one especially if you have the ability to draw.

An illustration skill comes as a very extra skill that can accompany design for example if you designing a advertising campaign, you could make it more artistic by illustarting some of the elements to create more visual appeal. 

As a member of both camps, what do you think can help a company decide which of the two they need?

Company should base on what kind of visual identity they would want to show and also their target market. Ideally, how you would want to present the information because you cannot use cartoonish illustrations to represent a more serious brand thus also picking a right illustrator to represent their image.

What advice do you have for younger visual artists or those with an interest in the field?

Experiment a lot and don't get tired of trying out different things until you find your style, your voice.