The difference is due to the clothes cut or length, hat shape, type (geometrical or floral) and colour of embroidery. One can write tones of pages about this subject only if you think at history, importance, diversity and beauty of the costumes.
Nowadays only in few regions, mainly in the north-west of the country, in Maramures and Oas peasants still wear their traditional clothes, especially on Sunday mornings when all the village participate at the religious ceremonies or at other specific events. Considering there are only few cloth-makers left trying to prolong this tradition, and the modernisation wave is very strong, it seems in few years these marvellous hand-made costumes will be available only in museums collections or at astronomic prices.
Photos source: Wikipedia, pictures originally downloaded by Marghiolita Rogojan and AndradaDaciana.
A bigger gallery of costumes can be seen on Flickr here, and details about the structure of traditional Romanian clothing, its history and evolution well documented and "punctuated" by over 350 pictures can be studied on "Traditional costume in Romania" by Eliznik (can be a little demanding, but it is definitely worth the effort).
Not to forget that Romanian traditional blouse was featured by Matisse in his painting "Romanian blouse", and used as an inspiration by few famous fashion designers.