Workshop is one of two rewards you can choose after you have upgraded a level 1 city to level 2 (the other being Explorer). It increases your city's resource production by 1.

The workshop sprite is identical to that of a level 1 Forge. It is considered to be one of the most useful early-game upgrades because of the boost in star production early on.

City Upgrades
Border Growth, City Wall, Explorer, Park, Population, Population Growth, Stars, Super Unit, Workshop