Blinds are used to block light from entering your windows and to also provide privacy from the outside world.
There are many kinds of window blinds available, common types are vertical blinds, roman blinds, roller blinds, venetian blinds and wooden blinds.
Yes, the window which you wish to install blinds into will need to be measured to ensure the blinds fit. Too small and they will allow light in and too large and they simply will not fit the space.
Curtains are fabric used to cover the window. In colder climates curtains can have an advantage in providing insulation from the cool weather coming through the window. Blinds are more used to block light out.