The main difference is the construction. in terms of materials scale and lamellar could be made from hardened leather, bronze, iron or steel.
Scale armour is attached to some kind of backing material, where as lamellar doesn't need backing material because the lames are attached to each other. Lamellar armour was very widespread up until the 15th century, many Middle-Eastern, Eastern European, Central Asian, Chinese and Japanese armours were of lamellar construction. For some reason though, it seems to have been quite rare in Western Europe.
Lamellar armour:
Click on the links for more pics of lamellar armour:
http://www.armourarchive.org/patterns/lamellar_templarbob/images/image12.gif
http://www.armourarchive.org/patterns/lamellar_templarbob/images/image21.gif
http://www.schmitthenner.com/images/SCH1577ac.jpg
http://i4.photobucket.com/albums/y110/Nephtys/Oriental%20Warriors/Mongol_Heavy_Cavalryman.jpg
http://i4.photobucket.com/albums/y110/Nephtys/Oriental%20Warriors/Tibetan_Armour_2.jpg
Scale armour:
http://www.gloryofdresden.com/modularbuilds/gobdwebimages/calloutimages/14--SCALE-ARMOUR.jpg
http://members.iinet.net.au/~bill/gallery/2000/armour.jpg
http://www.schmitthenner.com/images/sch1564.jpg