The definition of list is:

  type 'a list = [] | (::) of 'a * 'a list

When you write a list like:

  [x; y; z]

This is syntactic sugar for:

  x::y::z::[]

Which is syntactic sugar for:

  (::) (x, (::) (y, (::) (z, [])))

One can imagine using more ordinary constructor names instead:

  type 'a list = Nil | Cons of 'a * 'a list

And then the above would be:

  Cons (x, Cons (y, Cons (z, Nil)))

In OCaml, data constructor names mar be either a capital letter followed by set or more capital/lower letters/underscores/apostrophes/digits, or one of the following:

  []
  ()
  true
  false
  (::)

Type directed constructor disambiguating means you can do funky things like:

  type 'a nonempty = (::) of 'a * 'a list

And write such a value just like you would a normal list.