In essence, planning scheme ordinances and interim development orders are remnants of a nearly-extinct planning scheme started under the Local Government Act 1919. While the planning of much of the state is now governed by the scheme established under the Environmental Planning and Assessment Act 1979, there are still some bits of land for which plans under the new scheme have not yet been established and where the old scheme lingers on.
Under the Local Government Act 1919 scheme, planning scheme ordinances performed much the same role as local environmental plans under the Environmental Planning and Assessment Act 1979 scheme. They would cover most or all of a local government area and set out various matters related to the use of land, many of which bear a striking resemblance to components of the current standard instrument, such as a land use table, additional permitted uses, and so on.
Interim development orders were generally used to cover smaller areas within an area covered by a planning scheme ordinance. If you think of a bowl of soup with small droplets of oil on the surface, the planning scheme ordinance would be like the soup, and interim development orders would be like the oil, displacing the soup here and there.
A good example of how this old scheme continues, in some places, to coexist with the new scheme can be seen in the Ku-ring-gai local government area.
Ku-ring-gai's town centres are currently covered by a single standard instrument local environmental plan (that is, a local environmental plan under the Environmental Planning and Assessment Act 1979 which has adopted the standard instrument—principal local environmental plan set out in the Standard Instrument (Local Environmental Plans) Order 2006). The majority of the remainder of the local government area is covered by the Ku-ring-gai Planning Scheme Ordinance, with the exception of two small areas which are covered by interim development orders.
If, like me, you're a visual person, the Ku-ring-gai local government area looks roughly like this:
Ku-ring-gai Council has indicated that its planning scheme ordinance and interim development orders should be replaced with a new SI principal LEP at any moment, and presumably this is the case with other areas of land in local government areas or parts of local government areas still governed by these old-style instruments, but until then landowners will continue to have to navigate a perplexing series of planning instruments to work out what laws apply to land they are rightful owners of.
If it's all too much (and unsurprisingly, for most people it is), I recommend that you contact your local Council or the NSW Department of Planning and Environment for assistance.