Radiator caps

What is the difference between a vented and non vented cap. I bought a chrome cap and it wouldn't work. When the pressure got up it started leaking under the cap. I thought it was the stock radiator. But when I bought my aluminum rad it did the same thing. I put on the vented stock cap and it stops. It did this with other chrome caps so it wasn't a bad cap. They said they come non vented. I have a catch can but it leaks from under the cap. Thanks for any info to help me understand this.

Fred B