English lover is essentially correct. Different levels of formality but essentially they're synonyms. One thing pops in to my mind though - "intelligent programming" would be computer programming that seemed to have the ability to change to adapt to situations instead of following a set path, you could not then change, "intelligent" for clever or smart (well maybe smart it depends on which phrase has been accepted by the majority). In every day, normal/informal speech, "clever, intelligent and smart" are interchangeable. He's clever/intelligent/smart. But It's clever isn't it? How intelligent people can use their intelligence to do these impressive things! Yes, some people are very smart/intelligent/clever. You couldn't say it's smart/intelligent isn't it? So when describing a situation, the words are not interchangeable, but informally speaking about a person, they are. To keep it simple, I'm sorry for this long answer - just use the word, "intelligent" when you want to describe a person, "clever" is very informal and, "smart" is probably American. "Use your smarts" - is another hideous Americanism (hi Tim!) that deserves to be shot and buried. She/He is intelligent, it is/ they are / we are/ it was etc etc - intelligent - with intelligence - intellectually speaking. Informally with friends, "He's clever/intelligent/smart".

and bright,brainbox,smarty, brilliant , nimble ...and others..they all are synonyms , they differ in the level (formal,informal,and slang ), so choose the fitting word..