In my language (Portuguese) there is only one word to describe our cousin close to human being: macaco.
However, I've noticed that English people describes that animal mostly as monkey. But what about Ape?
Is there any difference? One word is less used than another one? Some of them are old-fashioned?
Jmat said it right. Basically the difference is taxonomical(biology). Apes are a taxon - superfamily. Monkey is a term for bunch of different species, which are not exactly connected, not a biological class.
Unfortunatelly, if you want to cover all species considered monkeys you have to use term Simians - sadly, this term also covers all apes.
I wouldn´t worry about it, nobody cares.
All apes are monkeys, but not all monkeys are apes.
Gorillas, chimpanzees, humans, orangatangs and gibbons are types of apes. If a monkey isn't one of those, then it's some other type of monkey.
It's very common to interchange the two terms colloquially though.
Apes and monkeys aren't the same.
This is a really nice explanation of the differences, with lots of cute photos: