There are a few different types, but the most common are:
The Photographer
If you are taking pictures, a friendly-looking local or couple will come over and offer to take the photo for you. Either you will be charged for this service to get your camera back, or they will run off with the camera. Another version of this is when someone asks you to take a photo of them. They hand you their camera, it doesn't work, so you go to hand it back, and they let it fall out of their hands. Of course it will hit the ground, and they will blame you for the damage. Normally, it is a small group that will start yelling at you for money to replace it or fix it. You can also get pickpocketed during this entire time.
Little Too Helpful
They notice you are looking at a map. They speak your language perfectly and offer to take you there. They might be scoping you out and planning a robbery (ex. they might ask what hotel you are at, if you need an ATM, etc.). If they take you to an ATM, they are either going to memorize your PIN for when they pickpocket you later or just run off with your cash.
Masseuse
You look tired or need to relax, so they will give you a massage. They will tell you it is free for a limited amount of time. except they won't stop and demand a very high payment. They will keep pestering you until you pay.
Extortionist
Typically, a female will seem like a normal lady (or be smoking hot) but will get you drunk so you will take her back to your room. Once there, you keep drinking, but she slips sleeping pills or GHB into your drink while you are not looking. You pass out, she cleans out the room, and when you come to, you have nothing. It can be worse than this, though. Some will take photos of you while you are naked and/or pose you in questionable positions. They will demand a payment to delete the pictures, or else they will send them to everyone on your contacts list.
English Leaner
A person loves the country you are from and wants to work on their English, usually over a pricy dinner or drinks that you are expected to pay for.