Unlike human society, which continues to progress both technologically and socially at an extremely fast pace, merfolk society has remained very much the same for countless centuries. While constitutional democracy is now the norm in many, if not most, places in the human world, merfolk kingdoms are still run as old-fashioned monarchies. Although the vast majority of mermaid queens and merman kings are very benevolent and fair rulers, there have nonetheless been occasional instances of tyrants gaining power.
Due to electricity obviously not working well underwater, there are no computers, televisions, or stereos in the mer-world, although magical "shell phones" are used in place of telephones. Even books and scrolls are relatively rare since it's pretty difficult to read and turn pages under the seas. The most popular forms of entertainment are oral storytelling and live concerts and theatre.
Music plays a particularly important role in merfolk society. While humans cannot speak, much less sing, underwater, the vocal chords of mermaids and mermen have specially evolved to work below the surface. Some kingdoms even use singing as the main form of speech. Almost all merfolk are good singers, but only some of them possess the dangerous magical ability of the "Siren", which means that they can, with the right training, use their beautiful singing voices to hypnotize just about anyone into doing anything.
As a species, merfolk possess incredibly passionate and loving hearts. When a mermaid or merman falls in love, they tend to love more truly and deeply than most mere mortals. They're essentially hard-wired to fall passionately in love only once in their lifetimes, which either means that they live happily ever after if their beloved reciprocates their feelings, or are doomed to never find romantic contentment if their affections are not returned. The only one who is always immune to the magic of a strong Siren is the one they love.
Another major difference between most human cultures and the culture of mermaids and mermen is their attitudes regarding clothing. Most mermen and some mermaids don't wear any clothing at all, other than perhaps a piece of jewelry or a hair accessory. While brassieres made of seashells, pearls, and seaweed are commonly worn by mermaids in some kingdoms, no one would ever be arrested for indecent exposure in the mer-world, as modesty is a human societal construct that most merfolk actually find quite hard to comprehend.
How merfolk view the human world varies depending on the kingdom and the individual. While all kingdoms have laws limiting contact with the surface world, some kingdoms' laws are certainly stricter than others. Some kingdoms encourage rescuing drowning humans, while others punish that with exile or worse. Many merfolk strongly resent the environmental destruction that humankind has wrought on the ocean and its creatures, but some are immensely curious about the world above the sea.