A zoning map divides a city or county into districts, each shown by a color or pattern and a short code, such as R for residential, C for commercial, I for industrial or A for agricultural, often followed by a number. The codes are local: two places can use the same letters for very different rules, so the code's text is the only reliable key.
Find the property, note its district, then look that district up in the zoning text: the use table for what is allowed, and the dimensional standards for setbacks, height, lot size and density. A lot split by a district line may be governed by two sets of rules.
Look for overlays. Many maps add overlay districts on top of the base zoning, for historic areas, flood zones, airports, corridors or design review, each adding its own rules. Planned development districts, often marked PD or PUD, follow their own approved plan rather than a standard district.
Use the official map. Online map viewers are convenient but often say they are not the official record, and maps change with each rezoning. Do not confuse the zoning map with the future land use map of the comprehensive plan, which shows what the government intends, not what the law allows today. In Charleston, South Carolina, for example, the city and the county each keep their own zoning map for the land they govern.