Further to the above, before words that begin with an unstressed syllable that begins with a 'h', such as 'historian' and 'hotel', you may (but don't have to) use 'an': 'an historian', 'an hotel' (or 'a historian', 'a hotel').
And there are also some words which will differ in different places: some people (such as myself) pronounce aich with a haich, so would say 'a http'; others don't. Americans, i have been led to believe, say 'herb' without the 'h', so an American might write "color of an herb," whereas a non-American might prefer 'colour of a herb'.