Interpart modeling is where you can do a couple of different things.
One example is to use an expression from one part to drive other part in the assembly. For instance I may have some value of diameter that can drive the parts for a shaft and cylinder from this value. That is one type where the expressions are linked between the parts.
The another type is to use geometry between parts. I can use a body from one part and make a copy of it to another part. I can use this body as a trim body. I can also just copy faces between parts it that is all that is needed.
Let me know if you need any more intformation or clarification on this.
That is a very open question as there a number of methods you could class as interpart modelling, one simple method would be using copy paste from one part to another or realting an expression from part to another, however you are probably referring to geometry linking, and simply put this is where geometry in one part is copied associatively to another, either for reference or as is often the case used when designing a new component. A simple example would be if you had an assembly structure for a bottle, the structure contains the top level assembly with the bottle and bottle cap as sub components. So lets say you start by designing the bottle component first, once it is finished you then need to design the bottle cap. Obviously you could do this in isloation by knowing what sizes you need e.g. the bottle neck diameter, height etc. , however it would make more sense if you could somehow reference the geometry of bottle neck design as an aid to starting the cap design to ensure the correct fit plus if the bottle changed then the cap would update accordingly. Interpart modelling inside NX allows you to do this with also a good level of control. You have tools such as WAVE Geomtery Linker which enable you to copy geometry from anywhere in the assmebly to the current work part and then re-use it in a new design. I suggest you take a look at the Interpart Modelling section in the Assemblies help documentation for more complete look at whats possible. If you have any further questions then just reply to me here.
Looking at the questions you are putting on this forum, can I assume you are a relatively new user? My suggestion would be to take an NX Design course which would cover many of the questions you are asking and would make your life working with NX more productive and enjoyable.