In Victorian England cookery writers used 'gateau' initially to denote puddings such as rice baked in a mould, and moulded baked dishes of fish or meat; during the second part of the century it was also applied to highly decorated layer cakes.
Judging by the amount of space given to directions for making these in bakers' manuals of the time, they were tremendously popular... The primary meaning of the word 'gateau' is now a rich and elaborate cake filled with whipped cream and fruit, nuts, or chocolate. Generally, the round cakes we know today descended from ancient bread. They were typically fashioned into round balls and baked on hearthstones, griddles, or in low, shallow pans.
Since the Second World War, however, usage of the term has honed in on an elaborate 'cream cake': the cake element, generally a fairly unremarkable sponge, is in most cases simply an excuse for lavish layers of cream, and baroque cream and fruit ornamentation...
The word gateau is the modern French descendant of Old French guastel, 'fine bread'; which is probably of Germanic origin.
Although both terms can be used for savoury preparations (meat cakes or vegetable gateaux) their main use is for sweet baked goods.
Cakes can be large or small, plain of fancy, light or rich.
Cakes can last much longer, some even improving with age (fruit cake).