Once a system of tags has been set up, it can be used to collect related information into special lists.
These commands all prompt for a match string which allows basic Boolean logic like ‘+boss+urgent-project1’, to find entries with tags ‘boss’ and ‘urgent’, but not ‘project1’, or ‘Kathy|Sally’ to find entries which are tagged, like ‘Kathy’ or ‘Sally’. The full syntax of the search string is rich and allows also matching against TODO keywords, entry levels and properties. For a complete description with many examples, see Matching tags and properties.
Chapter 6 of the manual
Sacha Chua's article about tagging in Org-mode