What are Form A contacts and Form B contacts in a photorelay? Also, what are A, B, and C connections? What is the difference?

<Form A contact, Form B contact>

For Form A contacts, the output side is turned on while a current at a certain level or higher flows at the input side. This type of contact is also referred to as "Normally open".
For Form B contacts, the output side is in the Off-state while a current at a certain level or higher flows at the input side, and the output side turns into the On-state when a current at the input side comes to a certain level or lower. This type of contact is also referred to as "Normally closed".

<A connection, B connection, and C connection>

In A connection, both AC and DC load currents can be handled, and in B and C connections,  DC load currents can be handled. In A connection, photorelays can also be used without AC signal switch and polarity. In C connections, since MOSFETs are connected in parallel, compared with A connections, DC load currents twice as large can be handled.