the ecotecs were the new update engines in the VS-VY, which Holden claim achieve more power while being more fuel efficient. the ecotec stands for something like 'emissions and consumption optimisation thru technology'. theyre a similar engine, holden made some changes to the buick 3800. the heads on the engines are different, et cetera.
whats been said already is pretty right, the ecotec is a redesigned buick, built with certain targets for the modern market in mind (after all, the original buick engine is very old). it had weight, size, performance, emissions and economy targets in mind when being built and as such has various changes such as those mentioned already and a few others like shorter conrods.
one of the best changes (i believe) is the heads, with the ecotec having a much better design and longer runners.
a few other differences to the ecotec are, higher compression (9.4:1 compared to 9:1), electronic sequential injection and the use of a maf sensor (as apposed to map).