sham means fake or false. So a sham marriage is not a real marriage. He is a fake doctor if he does not have a license. Sometimes they also use the word "quack" which you often hear in the news here in the USA. Charlatan is an old word that I rarely hear. I see it mostly in books but it has the same meaning as a fake, quack or fraud.

Both sham and charlatan are used to describe something/someone who is deceitful or a fraud, but I haven’t heard people referred to as “shams” often. It’s more common to refer to deceitful people as liars, frauds, or charlatans and to refer to things like marriages, businesses, or weddings as shams. My 2 cents, anyway.