In the beginning of the phb, the rules state that a critical success is made when both action die (on an action roll) both roll ten. It mentions that special effects happen when a critical success is rolled but I cannot seem to find these effects anywhere in the book. Any help or information on the topic would be great.

-Thank You For Your Time

It depends on the roll and usually the GM steps in to declare what effects take place.
Take for example, if you hit someone with your weapon and you critically hit, you would automatically land a wound on your target and the GM can flare up any other additional effects. For other rolls it can mean you performed a task so well that it grants you additional bonus lasting a couple of seconds, or a significant decrease in time performing the task, or even acquiring a tid-bit of very obscure information.

Hope this helps

Ah thank you! That makes sense!

However as there is not a canonical list of possibilities then if a player gets a crit then they should also, depending on their gm, either ask if there are any extras or suggest an extra effect. Feng Shui has it right when it suggests that anything that looks cool but has no other significant effects should be allowed (imho).