Definition:
Information Retrieval: Information retrieval (IR) is finding material (usually documents) of an unstructured nature (usually text) that satisfies an information need from within large collections (usually stored on computers).

Chapter 1:
first manually classifying some documents and then hoping to be able to classify new documents automatically.
Three prominent scale:

1. web search
   gather documents for indexing, being able to build systems that work efficiently at this enormous scale, and handling particular aspects of the web,
2. personal information retrieval
3. enterprise, institutional, and domain-specific search

Chapter 6: Scoring, term weighting and the vector space model
Term frequency and weighting:
a document or zone that mentions a query term more often has more to do with that query and therefore should receive a higher score.

Reference:
https://nlp.stanford.edu/IR-book/