As we know that induction cooker works on the principal of induction of current in a conducting plate. So I just wanted to know what will happen if we place salt water in a plastic container on the induction cooker will it get hot as it is also a conductor or nothing will happen?
The induction cooker works by passing a rapidly changing magnetic field through the thing to be heated.
That rapidly changing magnetic field creates an EMF (Electro Motive Force) field. (Faraday's law). This is like a voltage. Current will flow, proportional to the conductivity. No conductivity, no current. Low conductivity, low current. High conductivity, high current. Heat will be produced according to I^2 R.
So there will be a current in the salt water, and it will be heated. But not very much. Its conductivity is much less than copper or iron, so it will not have a very large current or amount of heating.
As for Iron or other magnetic materials, they might be part of the design, required to concentrate the magnetic field, but in general, induction heating is from currents induced in a conductor.
Many weeks later: Those darn induction cookers are advertising everywhere. Sure enough, their writeup says you need some magnetic properties to the cookware. Their explanation as to why, something like the first line in JJ Fleck's answer, is garbled and not exactly correct. The addendum of Fleck's answer has it spot on, and with references too.
Being a conductor is not enough, you also have to have some magnetic properties in order to induce current in some material. That's why not all pan "work" on an induction cooker.
Bobby Bennett's answer show me that I have been a bit too concise in my previous answer. Here is a more complete one:
In conclusion, you can for sure induce current in your salted water and increased it when increasing it's conductivity with ions but without the "doping" due to ferromagnetic properties, you would not be able to heat it with a usual "kitchen" induction system.