The reason for the dual name is due to the original brewer of the style, San Francisco based Anchor Brewing Company, trademarking the use of the term ‘steam beer’ when labelling their now famous brew.
This trademark meant that other brewers wishing to replicate the style had to choose a new name for their products, leading to the creation of the California Common.
Naming aside, these beers are now brewed right along the West Coast of America and beyond and are known for being medium-bodied, typically amber in their appearance, and moderately carbonated.
Flavourwise, these brews possess a moderately strong malt profile that imparts flavours of caramel and toast and a light to moderately intense hop profile that imparts a pronounced degree of hoppy bitterness and lesser amount of woody or spicy hop flavour.
For more on the history of the brew, read this article.