Table, view or collumn comments

Summary 
In the database dictionary, table or view and collumn comments are stored in views DBA_TAB_COMMENTS and DBA_COL_COMMENTS

SELECT OBJECT_NAME FROM DBA_OBJECTS 
WHERE OBJECT_NAME LIKE 'DBA_%COMMENTS%' 
AND OWNER = 'SYS';

OBJECT_NAME
------------------------
DBA_COL_COMMENTS
DBA_TAB_COMMENTS
DBA_OPERATOR_COMMENTS
DBA_INDEXTYPE_COMMENTS
DBA_MVIEW_COMMENTS

For example to see the comments for view DBA_ROLES:

SELECT * FROM DBA_TAB_COMMENTS
WHERE TABLE_NAME = 'DBA_ROLES';

OWNER          TABLE_NAME          TABLE_TYPE   COMMENTS
----------------------------------------------------------------------------------------
SYS                DBA_ROLES        VIEW         All Roles which exist in the database

To find the collumn comments of the view DBA_ROLES

SELECT * FROM DBA_COL_COMMENTS 
WHERE table_name = 'DBA_ROLES';

OWNER TABLE_NAME         COLUMN_NAME              COMMENTS
-----------------------------------------------------------------------------------------------------------
SYS      DBA_ROLES          ROLE                             ROLE NAME
SYS      DBA_ROLES          PASSWORD_REQUIRED  Indicates IF THE ROLE requires a PASSWORD TO be enabled

If you want to add comments in a table or collumn use the following syntax

COMMENT ON TABLE TABLE_NAME IS 'this is a test comment on table';
COMMENT ON COLUMN TABLE_NAME.COLLUMN_NAME IS 'this is a test comment on collumn';

You can drop a table comment, you simply do an edit to nothing

COMMENT ON TABLE TABLE_NAME IS '';