There are quite a number of different types of media an illustrator can use: pen and ink, watercolour, pencil, pencil crayon, pastel, acrylic paint, oil paint and gouache. Which media you use for a piece of work will depend upon the kind of work you are producing.
For instance, if you wanted to create a colour picture book with a soft look to it, then watercolour or pastel might be a good choice, wheareas if you wanted something bold and graphic, then you could consider creating it digitally.
Pen and ink is often used for interior illustrations for older children's books. Watercolour's transparency allows you to work over the top of it with other media, such as pencil crayon.
Acrylic paint is useful for creating work with striking tone. You might find it easier to work with some media than with others, and from this you may develop a favourite media to work with.