Citizens with no profession are automatically assigned to be laborers, however, when the player assigns one or more citizens to a particular profession they then take on that role. This is not permanent, so they can change professions given the need.
Whenever citizens with an assigned profession have no work to do (farmers during winter, builders with no queued construction work to be done, etc.) they do the same work that laborers do. As work becomes available citizens with professions automatically stop doing labor work and continue with their profession.