The derived SI unit of area is the square metre (m2) based on the SI unit of length. Originally the are equaling 100 m2 was the unit of area in the metric system with its prefixed versions decare and hectare. Today only the hectare is commonly used when measuring land areas.
A barn is useful on the level of nuclear physics being approximately the cross sectional area of a uranium nucleus.
Many of these older English units are derived from the amount of land could be ploughed or otherwise handled in a day or other time period. See the article on traditional English area units for more details. The conversions here must be treated as approximations.
The metric system introduced in 1924 has replaced the traditional Japanese Shakkan-hō (尺貫法) system but for example the tsubo is still used when discussing land prices. A tsubo is the area of two standard tatami mats.
The larger area units are used by farmers when discussing the sizes of fields.