Two pieces of data (with the constructor ::) with 1 being the head of the list (type 'a) and one being the tail of the list (type 'a list, a recursive definition).
:: is an allowed identifier in oCaml. If that is not allowed, the typical names are Cons for :: and Nil for []
Do you know Cons' meaning ?
Also why does :: act as a separator... ? Is that something you ca do with any Data constructor, I mean can I write type 'a = 'a Cons of 'a | Nil of 'a ?
As :: doesn't start with a letter, it's infix by default. But basically a binary constructor. If you want to create the List [1,2,3] in the prefix notation you might be used to: ::(1,::(2,::(3,[])))