Phone cameras are sold on megapixels, which is close to the least informative specification available.
Sensor size
The most important number and the one rarely advertised prominently. A larger sensor collects more light, which produces less noise, better low-light performance and more natural depth of field.
Expressed as a fraction like 1/1.3 inch. Larger fractions – smaller denominators – are better.
Aperture
How wide the lens opens, written as f/1.6 or similar. Lower numbers let in more light.
The iPhone 18 Pro introduced a variable aperture, which physically changes rather than being fixed – the first time a phone has offered genuine optical control over depth of field rather than simulating it.
Megapixels
Past about 12, the returns diminish sharply for phone-sized sensors. High-megapixel phone sensors typically combine groups of pixels into larger virtual ones anyway, producing an image far smaller than the headline number.
A 200MP sensor does not produce better photographs than a good 48MP one.
Optical versus digital zoom
Optical uses actual lens elements and loses nothing. Digital crops and enlarges, losing detail.
Manufacturers now quote optical-quality zoom, which is digital zoom from a high-resolution sensor. Better than plain digital, not equivalent to true optical.
Stabilisation
Optical stabilisation physically moves elements to counter shake. It matters enormously for low light and video, and more than most specifications.
The thing that matters most
Processing. Two phones with identical hardware produce visibly different photos. This cannot be read from a specification sheet, which is why sample images from reviews are worth more than the numbers.