What is the difference between point and multipoint ? linestring and multilinestring ? polygon and multipolygon ? In PostGIS
What is the secret behind defining "Multi" shapes?
"Multi" means, that several objects of that kind are "grouped" together to single object. For example, imagine Philippines. That country consists of lot of islands - you can't draw it with single polygon; and it would be inconvenient to store it as several different database rows, each containing single polygon.