There is no difference between the word "liter" and "litre". Both words mean the same thing, even though they have different spellings. It all depends on where you live as to which spelling is correct for you.
Most English speaking countries have their foundations on one of two language bases - American English or British English.
In Australia, the vocabulary and grammar is closer to the Bristish English model.
In American English, the preferred spelling is "liter".
In British English, the preferred spelling is "litre".