Common household wires are solid for 14, 12 and 10 gauge wire. 8 gauge and larger wires are typically stranded. Extension cords (often 16 or 18 gauge) are also typically stranded.
are more flexible and less likely to suffer strain if they are stretched. They are more workable, and won't break with repeated bending.
aren't intended to be coiled and uncoiled regularly, and therefore, are not suitable for extension cords. They can't take the considerable bending and flexing that extension cords typically receive.
Solid wire is also not suitable for overhead runs from the house to the garage, for example. Overhead wires that have to span considerable distances should be stranded, because they must be able to flex regularly. Solid wires should span no more than 4 1/2 to 5 feet between supports.