The Stanford NLP group defines coreference resolution as:

Coreference resolution is the task of finding all expressions that refer to the same entity in a text. It is an important step for a lot of higher level NLP tasks that involve natural language understanding such as document summarization, question answering, and information extraction.

Coreference resolution should not be confused with Named Entity Recognition, which is focused on labeling sequences of text that refer to entities–but not focused on linking those entities together.