Written as width to height, aspect ratio describes frame shape independent of resolution. 3:2 is the classic 35mm stills proportion. 4:3 is the older, boxier standard that survives in medium format and many compact cameras. 16:9 is the television and web video default, and 2.39:1 is anamorphic widescreen.
Shape carries feeling. Wider frames emphasise landscape, environment, and lateral movement, which is why epics are shot wide. Taller or squarer frames emphasise the figure and feel more intimate and more formal — part of why 4:3 has returned in art-house cinema and why 9:16 dominates phone-native storytelling.
It should be decided before shooting, not after. Composing for 3:2 and cropping to 9:16 usually means discovering you framed the wrong thing. If a project has to deliver in several ratios, the standard practice is to shoot the widest and protect the tightest — frame so that key action survives the most aggressive crop.