What is this AC source?
Yes.
No, they're just wires going to the source. 
It's a MOV (Metal Oxide Varistor) to absorb surges (through-hole types look like ceramic disk capacitors). 
Markings on the symbol for the AC input of a bridge rectifier. 
A bridge rectifier (four diodes). 
More markings on the symbol for the DC output of the bridge rectifier.