Light soy sauce is saltier, thinner and lighter in color. Dark, in contrast, is less salty, more viscous, and darker (obviously).

The primary function in most recipes is the color; dark soy sauce really changes the visual ppearance of food which can be good or bad depending on the dish.

Many folks confuse light with the low sodium "lite" soy sauce.

Freaky--I have been researching this all day and am still confused on the difference between dark soy sauce and sweet soy sauce. I'm just going to add molasses and call it a day/

Is dark soy the same as sweet soy (I don't think so)? I use kecap manis / sweet soy primarily for pad si ew, and occasionally when I'm lazy making teriyaki/bul go gi-type marinades (one ingredient instead of two)

Sweet soy sauce (kecap manis) is definitely not the same thing as dark soy sauce.

I just checked the labels on my various bottles. In comparison to light soy sauce, my dark soy sauce (Pearl River Bridge Superior Dark Soy), in addition to being darker and thicker and less salty as noted above, has sugar added to it. (My bottle of sweet soy sauce also has sugar added, but significantly more than the dark soy sauce, and it is also much more viscous.)

Dark soy sauce （老抽）and light soy sauce （生抽） are used in different ways in Chinese cuisine. Dark soy sauce is used during cooking for its color, as it gives a wonderful caramel color to braised dishes, in particular braised pork belly or braised eggplant. Light soy sauce is used for its more robust flavor, it's saltier. Chinese kitchens will have a bottle of each, and many recipes use a mix of both.

From my understanding, light soy is basically akin to a "first-ferment", and dark soy is that further fermented and sometimes added to for a deeper finish.

Light soy is salty, like fish sauce without the fishiness. Dark soy is less salty with a flavour which is much like it's been caramalised-- slightly sweet, almost bitter.

Light soy can generally be used where salt would be without (much) changing the nature of the dish. Dark soy is generally unsuited as a substitution for salt and is more commonly used for it's colour and bittersweetness in braising.

Both are also commonly used as dipping sauces and their different flavour profiles lend completely different treatments to the same foods. Example, steamed chicken dipped in light soy takes on a fragrant umami-ness. The same dipped in dark soy (stirred into chilli ginger garlic sauce perhaps) elevates the chicken into an otherwise unplumbed meatiness :)

My familiarity with molasses only extends to baking and I can assure you that it is in no way a substitute for either soy sauce in spite of it being sometimes added in the production process for dark soy.

I'm sorry I can't be any clearer as all this is just personal observation and colloquial information, but I hope it helps in some way.

Light soy sauce is saltier, and is suitable for quick marinades (10-20 minutes), or addition at the end of the cooking time. For instance when marinating meat for a stir fry, or drizzling over steamed fish. You only need a small amount.
Dark soy sauce is slightly sweet, more aromatic, with a less salty taste. In Chinese cuisine, it is generally used for stewed/braised dishes such as soya chicken or red-braised pork.
Both can be used for dipping, depending on the dish and personal preference.

I actually asked this question because I wanted to try making pad-see-ew but I was unable to find kecap manis so I used "Thai black soy sauce" instead and I'm not sure it was the right choice. It turned out tasty but didn't taste quite like the pad-see-ew I'm used to. I'm guessing kecap manis is not the same as black soy sauce? Thanks everyone!

@ LEILANI - It's definitely not the same. Also, there's way more variations of soy sauce than just Dark and Light. Kecap Manis is flavored with garlic, star anise, and a lot of palm sugar (why its sometimes called "sweet soy sauce).

Try mixing some normal dark soy sauce with brown sugar, garlic, and star anise and simmer to get a substitute.

There are many recipes for kecap manis (aka sweet soy sauce) online. I made mine with soy sauce, dark brown sugar, and star anise (the recipe already had copious amounts of garlic and ginger, so I skipped those).
I also needed dark soy sauce, so I added molasses to some soy sauce, and the dish turned out well.
I have a seemingly well-stocked Thai market near my house, but I find some of the products to be of low quality, such as palm sugar that tasted moldy and many soy sauces containing sodium benzoate. So, I am happy I was able to find modifications online that worked.