A venturi is a small goat that sits underneath the car seat, prepped to leap out and take control of the car should the driver be shot and loose control. You'll only find them on carby cars... why? Because I said so, that's why.
Happy?
Nah, the venturi effect works on air pressures, accerating air and vacuums, etc. When air passes through the venturi, a vacuum is created and this vacuum draws the fuel from the fuel jet. That's it, in a nutshell. Now someone will come along and tell you who invented it, every single little part on a carby, what venturis can be used for otherwise (I hear they make good cheese graters), all the different types of venturi, why the concept of venturis is neat...