What is the difference between Linestring and multiline string, and things like compound curve, curve polygon, multicurve, and multisurface?
Any geographical object can be made of a random number of 'sub-parts'.
That applies for point, line or polygonal objects.
When you click in the attribute table you select only one object (=one row) but its geometry can be made of several parts (these sub-parts being contiguous or not ...)
You can check the multipart objects of a given table using (for example) the num_geometries function in QGIS
(by adding a calculated field using the following syntax :