loud is just an adjective to describe something or someone that is noisy. For example, "the music is very loud" You will here "loud" being used to describe a very eccentric, brightly coloured outfit as well but this is rare and a bit old-fashioned. Mainly older people will use this word in this way.
and loudly is an adverb describing the way a verb is done, but unlike aloud, which means to read something to someone else, loudly means with more volume,"say it loudly" is a bit different to "say it aloud". aloud is also an adverb, but means audibly, not necessarily with great volume. For example, "Please read the book aloud to the class."
I will try and use both in a sentence for you:
"Tim was reading a book and Sarah asked him to read the book aloud. Tim read too quietly, so Sarah asked him to read it loudly."