Today, I bring you....
origin:  Greek; dēmos= 'the people' + onuma= 'name'.
I am an American because I was born in and live in America. I'm also a Californian. And one can even say San Franciscan, due to my city or residence.
These ways of labeling people by location is referred to as a denonym, it denotes where they are from. There can also be slang (or "calloqual") words of the same relations, like New Zealanders being referred to as "Kiwis" or Canadians being "Canooks". Noteably, these type of words are capitalized.
Some rules for these labels (from Paul Dickson's Labels for Locals):
If the place name ends in -a or -ia, add -n (Californian).
If the name ends in -i or sounded -e, add -an (Hawaiian).
If the name ends in -on, add -ian (Oregonian).
If the name ends in -y, change the -y to an -i and add -an (Albany --> Albanian).
If the name ends in -o, add -an (Chicagoan).
If the name ends in a consonant or a silent -e, add either -ite or -er, depending on euphony (agreeableness of sound) (Mainer, New Hampshireite).
If the name ends in -polis, change that to -politan (Minneapolitan).
Naturally, there are exceptions: Utah, for example, ends in a consonant, but state residents are called "Utahns".