As Gerard says it's British English v American English. In every day life most places will accept either. Here are some further examples (British first): - grey/gray - colour/color - cheque/check (we Brits use the second as a verb, but the first as a noun), so you could check the details on a cheque for example.

Thank you Gerard! Is it the same for centre/center?

"Liter" is the spelling used in American English. "Litre" is the spelling used in Standard English.