Social has over 500M hits in NGrams, as opposed to only 7M for the more recent societal. So the main difference is OP probably always wants to use the former, because that's the standard word and it covers all meanings.
Societal is the more recent word for of [human] society, which is its only meaning. It's primarily used in academic writing, so OP is unlikely to need it.
This is a difficult question to answer, because both aches and pains are subjective experiences - like colours - which you're unable to share, but assume everybody understands. I would have assumed that every language has words for both ache and pain, so a dictionary would tell you the difference in an instant. But, I would also assume you've done that, so your native language might not (I'm interested to find out what language that is).
An ache is a persistent discomfort, typically dull so that you can try to ignore it, but sometimes all-encompassing, yet not sharp enough to describe as pain. Your legs would ache after a tough run; you would not describe this as pain. You usually get a headache, not a head pain. You would suffer pain when you cut your finger, then experience an ache as the wound heals.
A pain is something more localised, often (but not always) short-lived, and something you'd be less able to ignore.
When you receive an injection, there is a pain as the needle goes in. During the following days, the surrounding area will ache.
There is considerable overlap between the two, and it would be quite acceptable to say "the ache in my shoulder is painful".
Poets and songwriters quite often speak of their heart aching. This fits well with a persistent sense of yearning or melancholy. If they said there was a pain in their heart, it would suggest a quite different emotion.
Describing pain and discomfort is difficult and subjective; I imagine in any language. When a doctor asks you how much something hurts, how can you explain in a reliable way?
Best Answer
From my experience in the American metal fabricating industry, I would say that the two terms are often used interchangeably, but not always.
A wooden platform for stacking and moving product with a forklift or pallet jack could be called a pallet or a skid. They are equivalent when speaking generally without reference to a specific sub-type: "How many pallets/skids will fit on that truck?"
A steel item of similar shape and purpose is almost always a skid or a metal skid. Those would typically be for in-plant use because of their expense and great durability.
A plastic item of this class would usually be called a pallet or recyclable pallet. Recyclable pallet can actually mean two different things: a plastic or wood platform that is supposed to be returned to a supplier to be used again, or a wooden pallet that was salvaged, repaired, and resold at a discounted rate.
The tool pallet jack is common, but skid jack is almost unheard of.
Pallet can be be made into a verb as palletize, but skid rarely becomes skidded.