All of these, Cultivator or Tiller or Rotavators are labour saving, powered machines that break up the ground or prepare it for planting and cultivation.
They do this either by dragging or pushing rotating blades, tines or teeth through the ground churning up the ground as they rotate.
The term cultivator is generic and used to cover tillers and rotavators of all kinds, and in essence they are all forms of mechanical rotary tiller though some can have other cultivating attachments fitted such as ploughs.*
But there are differences within those terms.
A Front tine tiller breaks up already cultivated ground ready for planting or to create a fine tilth, while a Rear Tine Tiller can do the above plus break up non-cultivated, or non-sowable land, a lawn perhaps, to create soil beds.
* MD FACTFILE: The first powered, pedestrian rotary tillers were invented by Australian Arthur Clifford Howard in 1912, produced for sale in 1923, and Howard Australia Pty Ltd, a direct descendant of the original company, now owned by a New Zealand company, still make Rotavators, which are powerful, wheel driven rotary tillers.
The name Rotavator is a trademark of the Howard group.