What is the difference between the colours mauve and lilac?

http://www.nom.org/articles/humor/color.htm gives a man's guide to colours and has the following: -

Lilac = Light purple, but not lavender
Lavender = Light purple, but not lilac
Orchid = Purple
Violet = Purple
Grape = Dark purple
Indigo = Very dark purple
Mauve = A subdued pinkish grayish purple-like colour. or, light pink with a silvery quality.

From an Internet webpage point of view, there is none.
Both lilac and mauve are acceptable colours (color=name of a colour) as far as HTML code is concerned.
This demo below "should" show exactly the same.

LILAC

MAUVE

If entered into a Frontpage Express page in the color=name format, it automatically changes both to the same precise hexadecimal colour code of "#00C7D9"