Models
Idealised models usually consist of a simple intermolecular pair potential, whose purpose is often to study underlying physical phenomena, such as generalised phase diagrams and the study of phase transitions. It is entirely possible that a number of the models bear little or no resemblance to real molecular fluids.
Realistic models are usually variants of idealised models, parametrised to study a specific atomic or molecular system with a certain degree of realism. These models are usually designed to reproduce a subset of experimental physical properties, thus a certain model may be more suitable to a particular study, whist being wholly inadequate in the study of a distinct question.
Force fields consist of (hopefully) transferable parameters for molecular sub-units. They are designed to be applicable to a variety of molecular systems, over a range of thermodynamic conditions.