As nouns the difference between thief and sneak
is that
thief is one who has carried out a theft while
sneak is one who sneaks; one who moves stealthily to acquire an item or information.
As a verb sneak is
to creep or go stealthily; to come or go while trying to avoid detection, as a person who does not wish to be seen.
As an adjective sneak is
in advance; before release to the general public.