The document discusses various information retrieval models, including: 1) Classic models like Boolean and vector space models that use index terms to represent documents and queries. 2) Probabilistic models that view IR as estimating the probability of relevance between documents and queries. 3) Structured models that incorporate document structure, including models based on non-overlapping text regions and hierarchical document structure. 4) Browsing models like flat, structure-guided, and hypertext models for navigating document collections.