First Project with the Arduino - Sous Vide - Dimmer / Heater

So how does a PWM output work with a SSR? Is it chopping up the AC like per cycle or just turning it on and off so many times per second?
I get a little confused when it comes to AC cycles.

PWM is useless with 60Hz mains voltage if you don't know where the zero-volt-point in your cycle is.
You have to pulse exactly after this point, or you would just end up with a flickering light (or uneffective heater).