When it came to giving the German translation for "the server" the answer I gave was "der Server" however I got it wrong apparently that answer was der Kellner. However the German translation for Kellner is "waiter" in English. I would like to know why I got this wrong. Please explain.
"der Server" refers to the computer program/device while "der Kellner" refers to a waiter. So the translation really depends on the sentence/context.
Server is a widely used term for waiter or waitress in English.
Der Server is, as amorundpsyche correctly points out, a computer term.
The two are homophones in English but only the computer term has been borrowed by German.
The ubiquity of homophones in English makes in difficult to ever really know how to translate anything without context. Perhaps I exaggerate - a little. :)