Availability splits along a clear line: big cities with mobile international populations have deep furnished markets aimed at expats, students, and corporate tenants, while suburban and family-oriented markets skew heavily unfurnished, and in some countries unfurnished means literally bare, occasionally without even light fixtures or kitchen appliances. Ask locals what unfurnished includes where you are, because the definition genuinely varies.
Furnished itself is an elastic word. At minimum, expect major furniture: bed, wardrobe, sofa, table, plus white goods like fridge and washing machine. Fully furnished or equipped should add kitchenware, curtains, and lamps, while serviced or turnkey rentals include linens and sometimes cleaning. Semi-furnished, common in several markets, typically means appliances and fixtures but little furniture.
On economics: furnished units rent at a premium and often come with shorter, more flexible terms, which suits stays under a couple of years; beyond that, unfurnished plus buying secondhand furniture usually wins financially.