Schema object
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A schema object is a logical data storage structure. Schema objects do not have a one-to-one correspondence to physical files on disk that store their information. However, Oracle stores a schema object logically within a tablespace of the database. The data of each object is physically contained in one or more of the tablespace's datafiles. For some objects such as tables, indexes, and clusters, you can specify how much disk space Oracle allocates for the object within the tablespace's datafiles.
There is no relationship between schemas and tablespaces: a tablespace can contain objects from different schemas, and the objects for a schema can be contained in different tablespaces.
Associated with each database user is a schema. A schema is a collection of schema objects. Examples of schema objects include tables, views, sequences, synonyms, indexes, clusters, database links, snapshots, procedures, functions, and packages.
Adding items to the Schema, also called "extending the Schema", or even modifying existing objects can be a tricky business, and if done without proper knowledge, can be very destructive to your existing Active Directory infrastructure. This is because the Schema is a forest-wide setting, and any additions or changes to the Schema will be immediately replicated to each and every Domain Controller in each and every domain in your AD Forest. You cannot make any changes to the Schema and yet keep it within your domain's boundaries. Furthermore, changing existing attributes (such as configuring an attribute to replicate itself to the Global Catalog) will cause a forest-wide replication of all the attributes and objects, even if your change was just made on one attribute. Note that this behavior was changed in Windows Server 2003, but even so, you might unintentionally cause a major network load and a lot of overhead by simply clicking one one small checkbox on one small attribute.
