Paint consists of three or four components: binder, pigment, solvent and optionally filler.  What defines an oil based paint, is that it uses oil derived solvents (white spirit). There are many different oil based paints, alkyd paint would be the most common (alkyd is a binder).

Oil paints contain more VOCs that water based paints which can be harmful for human health. Therefore in some countries the use of oil based laquer for professionals is only allowed for application outside. For every oil based paint there is usually a water based alternative that is also of good quality.