A schema is a collection of database objects like tables, views, indexes, sequences, synonyms, functions, procedures, triggers, and packages that are associated with a particular database username. The document defines these database objects, explaining that a schema organizes objects for a username, views provide customized data presentations, indexes improve search performance, sequences generate integer values, synonyms are alternate names, functions and procedures are stored code segments with functions returning values and procedures not, triggers run on database events, and packages encapsulate variables and functions/procedures.