Ethylene Glycol is moderately poisonous stuff that is the most common and best working antifreeze. It is clear, odorless, and syrupy, and has a slightly sweet taste. Usually you will not be able to taste it in coolant though as it is out flavored by bittering agents to prevent humans and animals from drinking it. Ethylene glycol does break down over time and is why coolant doesn't last forever. Just in plain air, it will breakdown within a few weeks.
Propylene glycol (or methyl ethyl glycol) is just like ethylene glycol but doesn't do as good a job as coolant and isn't toxic.

Water covers 2/3rds of the planet. It's best to use distilled/deionized water, but it's actually suitable to use tap water if you know the mineral levels and have a tap water safe coolant. What you don't want are minerals in your cooling system, and the requirements for tap water by BMW are: clear, without suspended matter, pH from 6.5-8.0, less than 357ppm calcium, less than 100mg/L Chloride, and less than 100mg/L sulfate. Some manufacturers suggest even lower calcium levels. Calcium is bad because when dissolved calcium heats up, it precipitates out of the water as sand or forms as scale on hot surfaces. Ok, so now you know about 97% of what's in coolants.

Inorganic Acid Technology (IAT)
Conventional coolant that has been used for the past 60+ years made of inorganic salts, which sacrificially protect from corrosion not too different than a zinc plating on a steel bolt. The bad thing is that with hard water and/or time, these additives get used up and turn into sand, literally, hard, rocklike sand, which will grind away any seal or bearing exposed to the coolant. Usually a 2 year lifespan.
-Silicates good for preventing aluminum corrosion but precipitate with use.
-Phosphates good for preventing steel corrosions but precipitate with hard water.

Organic Acid Technology (OAT)
Doesn't use silicates and phosphates but instead use carboxylate, sebacate, or 2-ethylhexanoic acid (2-EHA). Beware of 2-EHA, because it is a plasticizer, which means it softens plastics. Usually a 3 year lifespan and slightly more expensive than inorganic acid technology.

Hybrid Organic Acid Technology (HOAT)
Same as Organic Acid Technology, but usually with a small amount of silicates or phosphates to extend the useable life without the problems. Usually Japanese manufacturers add phosphates from past problems with silicates and water pump seals, while european manufacturers add silicates due to commonly hard water in Europe. This is the primary coolant used by manufacturers due to the extremely long service life. Usually a 4+ year lifespan and slightly more expensive than organic acid technology, but the cheapest when used with extended drain intervals.