Usefull Tables

DBA_OBJECTS
OBJECT_NAME
OBJECT_TYPE ->table,index,sequence,table partition,procedure,pakage,view,function,synonym,job...
CREATED
LAST_DDL_TIME
STATUS ->valid...
...and some other columns
DBA_SEGMENTS
SEGMENT_NAME ->table name or other
SEGMENT_TYPE ->nested table,index,table,cluster
TABLESPACE_NAME
BYTES ->size of that segment(for exmaple size of the table)
...and some other columns
DBA_TABLESPACES
TABLESPACE_NAME
BLOCK_SIZE
STATUS ->online,offline...
CONTENTS ->undo,permanent
LOGGING ->logging,nologging
RETENTION ->not apply,noguarantee
BIGFILE->no,yes
...
v$log

GROUP# ->group number
SEQUENCE# ->sequence number
BYTES ->size
MEMBERS ->amount of members
ARCHIVED ->archived or not
STATUS ->active,current,inactive
.....
v$logfile
GROUP# ->group number
STATUS ->
TYPE->ONLINE,OFFLINE
MEMBERS ->destination of the log member

v$sql
SQL_TEXT
SQL_FULLTEXT
SQL_ID
FIRST_LOAD_TIME
....
v$session
SADDR
SID
USER#
USERNAME
STATUS->active,inactive
SERVER->shared,dedicated
SCHEMA#
SCHEMANAME
OSUSER->operating system user
MACHINE->name of machine
TYPE->user,..
SQl_ADDRESS
SQL_ID->id of executed/executing sql value you can find in v$sql data dictionary view
PREV_SQL_ADDR
PREV_SQL_ID
LOGON_TIME

DBA_TABLES
OWNER
TABLE_NAME
TABLESPACE_NAME
CLUSTER_NAME
STATUS->valid,invalid..
PCT_FREE
PCT_USED
LOGGING
BACKED_UP->N/Y
NUM_ROWS
LAST_ANALYZED-> date
PARTITIONED->NO/YES
TEMPORARY
NESTED
ROW_MOVEMENT
SKIP_CORRUPT->enabled,disabled(if there exists corrupt block this option allows to skip it during query,if there exist and the value of this column is disabled than there will be ORA-01578: ORACLE data block corrupted )
DROPPED