I don't know much about early US history, I think I'll dig in, because having democrats being pro slavery and republicans anti slavery is something I wouldn't have expected.
The Democrats used to be pretty racist; before the American civil war, they supported slavery, and afterwards supported segregation, while the Republicans generally opposed slavery (Lincoln was a Republican, for example). As a result, the US South used to be heavily Democrat, and the north was heavily Republican. They basically flipped in the 40's and 50's due to the Democrats support for civil rights and the "southern strategy" (basically: appeal to racism, sometimes subtly, sometimes not, to get votes in the South) https://en.wikipedia.org/wiki/Southern_strategy
Today's (US) Republican party is nothing like the Republicans from 100+ years ago. It's kind of sad.
In the US, we had six party systems since the revolution. Each time we translation, we get two mostly new political parties which sometimes reuse the names of the defunct parties. Wikipedia has a good description: http://en.wikipedia.org/wiki/Party_system#United_States
you (GP) could also check out the XKCD of american political history. it has quite a few of the party names that people used throughout (which are entertaining, in a geeky kind of way):
Thanks you all, this has been very instructive. I realize I had a massive misinterpretation about left and right in the US.
That's because, in France, left wing has always been associated to humanism (humans first, economics then) and universal rights (the whole world deserve to have rights), while right wing is most about economics and nationalism.
From what I understand, US left wing is historically about equality for the "real USians" (kind of nationalism of the poors) while right wing, until getting own by far right in XXth's end, was for a very liberal society, be it in term of economics or in term of social rights.
This would explain a lot in the recent NSA reactions, where we were very shocked that the debate seemed to focus on whether or not US citizen were spied, rather than if mass surveillance was a problem at all.
I don't think the parties have changed much. The same moralist convictions that caused Republicans to be anti-slavery in the past are the same ones that make them anti-abortion today, for example.
I don't know much about early US history, I think I'll dig in, because having democrats being pro slavery and republicans anti slavery is something I wouldn't have expected.