As already answered, one spelling is British, the other spelling is American, but if you're in a professional setting, stick to one spelling; when I was in working in the UK, my british colleagues were always teasing me because of my "poorly misspelled" emails, so yea, in a short time I learned to write British English and stuck to that while in the UK; when I came back in the USA and i kept on using the British English, my american colleagues said, "dude, you got some spelling mistakes." :D