Surfactants, which do most of the work
Updated
The active ingredient of nearly every cleaner, and the one people spend least time understanding.
What a surfactant does
A molecule with a water-loving end and an oil-loving end. It lowers surface tension so water wets a surface, lifts oily soil off it, and holds that soil in suspension so it rinses away instead of redepositing.
Which is the actual mechanism of cleaning: soil is removed, not destroyed. It is also why a good cleaner needs no antimicrobial claim to work.
Four families
Anionic, negatively charged. Strong detergency, high foam, cheap. The backbone of most cleaners and of nearly all dish and laundry products. Sensitive to hard water.
Nonionic, uncharged. Excellent on oily soil, low foam, tolerant of hard water, and they play well with everything. Usually the workhorse of a hard-surface cleaner.
Cationic, positively charged. Weak detergency, substantive to surfaces, and many are biocides, which is exactly why using one thoughtlessly walks you towards a pesticide question.
Amphoteric, charge depending on pH. Mild, good foam boosters, good at extremes of pH, more expensive.
The rule that saves batches: anionic and cationic surfactants neutralize each other. Put them in the same formula and you get a precipitate and a product that cleans less than either alone.
Choosing one
Start with what the soil is. Oily and greasy soil wants nonionic; particulate soil and general detergency want anionic; a blend covers both, which is why most real formulas use two or three.
Then the foam you want, which is an appearance decision as much as a performance one: customers read foam as strength, and a rinse-free floor cleaner must not foam.
Then hard water tolerance, which is where a builder or a chelator comes in.
What they cost you
Surfactants are the expensive part of a cheap product and the cheap part of an expensive one.
They are also where the environmental questions concentrate, and where a supplier's documentation matters: biodegradability data, and whatever you are going to need for a Green Guides-compliant claim.