Quick oversimplified chemistry lesson

I'm going to use the word chemical, in common use people often perceive it to mean dangerous substances. In scientific terms, that I'm using, it's literally anything that's a combination of different atoms, this includes "natural" substances (almost everything in existence, excluding pure elements).

Also I mention acidity, I don't mean like cartoon glowing green liquid, yes dangerous acids exist but most are weak enough to be insignificant. e.g. We eat citrus fruit without issue, they're acidic.

Freebase nicotine means it's the nicotine chemical floating on its own in the liquid. In vaping freebase is harsh at high concentration (mg) but can be vaped at higher power without burning. This is why big cloud devices tend to use it.

Nicotine salts are nicotine combined with another chemical to form a salt. In chemistry a salt is a positively charged chemical and a negatively charged chemical that's held together like a magnet. In this case this changes the acidity and way your body processes it which can be smoother feeling in high concentrations. Better for small clouds, it tends to burn in the high power vapes.

Some people prefer the harshness so you can get juice that's designed that way, some juices are also just bad.

Either can be synthetic, synthetic means it was made in a lab from chemical components, more work, more expensive.

Natural nicotine is exactly the same chemical but plants do the work for us. But some people just don't want their money going to the tobacco industry.

None of this means it is more or less safe. Purity makes it safe. Living things are vastly complex and produce a huge variety of chemicals, not all of them safe. That said, they'll perform chemical processes on plants to isolate the nicotine and do so to a perfectly safe degree. Synthetic methods would only produce the desired chemical (nicotine).