Well, the simplest definition would be to say that this is a person who deceives others by pretending to have special skills or knowledge. These are usually guys who claim to know some secrets about medicine, but more often than not it is not true.
In English the word charlatan has a few synonyms such as a quack, a witch-doctor, or a medicine man. The word charlatan is not so popular in everyday life as this word has some negative associations, but I have a feeling there are quite many people who offer different sorts of alternative treatments although they have no idea about medicine. Of course they would never call themselves charlatans, but I know they are :)
In Polish, we have a similar word for a charlatan and it is 'szarlatan'.
OK, what's your standpoint on charlatans then?