The oldest reported version, from around 1913, includes Creme de Violette, a sweet violet-colored liqueur made from violet petals. Other old versions do not include it, but it is not clear why not - are they actually older versions and the Creme de Violette a later addition?
Everyone agrees that the base cocktail in any event is gin (I particularly like NH's Karner Blue Gin and it's a wonderful showcase for an Old Tom gin) and lemon juice with maraschino. Many today leave it at that, and that is a fine cocktail. The truth is that the Creme de Violette can be hard to find, and the cocktail is tasty without it.
But if you can find the Creme de Violette (or the pricey French brand Creme Yvette), I urge you to try it for yourself. I find it an improvement, both in flavor and appearance. The recipe on this site includes the Creme de Violette, but you can make the other version by substituting more maraschino for the Creme de Violette (you need the sweetness to balance the lemon juice).
Another way to look at it is that it is two drinks in one: make it one way in the spring and another in the summer, or one way to complement one kind of party, or one way to complement what you are reading.