In addition to representing documents as language models, we can also represent the topic of the query as a language model. In this case, the intuition is that the language model is a representation of the topic that the information seeker
had in mind when she was writing the query. This leads to three obvious possibilities for retrieval models based on language models: one based on the probability of generating the query text from a document language model, one based
on generating the document text from a query language model, and one based on comparing the language models representing the query and document topics. In the next two sections, we describe these retrieval models in more detail.