This is another area where there is some confusion about science and terminology.
All Purpose Cleaner
These will all be water based. They tend to be blends of solvents (both water soluble and otherwise), surfactants (which are cleaning agents, foam boosters, emulsifiers, suspending agents, etc.) and pH modifiers. Believe it or not, it tends to be the pH which has the biggest influence on cleaning. The majority of soils we encounter will be acidic in character so effective removal requires a high or alkaline pH. The other ingredients tend to help enhance this but they are rarely the primary agents. As Bunky noted, one must be cautious because some APCs can react with certain metals and even some plastic trims. The main reason this occurs is the alkalinity provider that is used. The most common problem is when caustic hydroxides are used (sodium and potassium mostly) - these are extremely effective agents, even at very low levels. They are also extremely easy to formulate with and are very cheap. Unfortunately they will quickly react with the likes of aluminium to form white residues which are chemically bonded to the surface (i.e. short of abrasive polishing, they are almost impossible to remove). They are very reactive with any oils so they tend to dry out plastics and they also react with glass to form crystalline silicates (you won't see this with brief contact, but repeated use can result in a slight haze on glass surfaces). There is no necessity to use these ingredients but alternatives (silicates for instance) which are safer will tend to be more difficult to formulate with and will routinely be more expensive. As Bunky says, you do need to be careful.
Degreasers
These can be either water or solvent based. Water based products are more or less interchangable with the above APCs. They use the same ingredients and formulation principles, they just have a different name. They are not any more dangerous than APCs. As with APCs, you just need to dilute them appropriately and, like with APCs, you need to be mindful of the potential for the presence of the noted caustics.
Of course there are then the water immiscible degreasers or solvent degreasers. These will tend to be blends of water insoluble solvents and surfactants. You should note that the alkalinity is not of significant note here! Solvent based degreasers will actually be a lot safer to use around metals than water based - the water based degreasers will often have corrosive character thanks to their high pH whilst solvent based products will not. These types of degreaser are what will be used out in industry because they are much more effective - grease dissolves in the solvent as easily as sugar would dissolve in water. Unfortunately they are much more expensive and cannot be diluted with water. These cannot be substituted into place with APCs.
A final little note with regards to pH. pH is a measure of the concentration of hydrogen ions in aqueous solution. I see many people on UK forums who discuss pH of solvent based products. Some will even stick pH meters into such products and state the pH. This is nonsense - solvents and solvent based products do not have a pH. If you measure something with a pH meter, it is meaningless, it is not a pH and worse still it is likely to damage your pH meter. Worst of all I have seen at least one major detailing manufacturer doing this with one of their solvent based products!
All the best
