Segmentation divides a heterogeneous market into groups that respond differently to marketing. The classic bases: demographic (age, income, family stage), geographic (metro/tier-2/rural — unusually powerful in India), psychographic (values, aspirations, lifestyle), and behavioural (usage occasions, loyalty, price sensitivity, benefits sought). The best segmentations usually lead with behaviour or need, then use demographics to find the segment in media.
A segment is only useful if it passes five checks: measurable (can you size it?), substantial (worth serving?), accessible (can your channels reach it?), differentiable (does it respond differently from other segments?), and actionable (can your mix actually serve it?). Segments that fail these are astrology, not strategy.
Targeting is the investment decision. Given your segments, where do you play? Options range from concentrated (one segment, all-in — the challenger's usual best move), to differentiated (several segments, tailored mixes — expensive, for scale players), to mass (one mix for all — earned only by category kings like salt and matchboxes).
Positioning then plants the flag in the chosen segment's mind — the subject of its own concept page. STP's contribution is making positioning answerable: you can't decide what to stand for until you've decided for whom.