What is inside X$tables in Oracle

The below mentioned list may not be complete or accurate, but represents an attempt to figure out what information they contain. I suggest not write queries against these tables as they are internal to Oracle, and Oracle may change them without any prior notification.

X$K2GTE2

Kernel 2 Phase Commit Global Transaction Entry Fixed Table

X$K2GTE

Kernel 2 Phase Commit Global Transaction Entry Fixed Table

X$BH

Buffer headers contain information describing the current contents of a piece of the buffer cache.

X$KCBCBH

Cache Buffer Current Buffer Header Fixed Table. It can predict the potential oss of decreasing the number of database buffers. The db_block_lru_statistics parameter has to be set to true to gather information in this table.

X$KCVFH

File Header Fixed Table

X$KDNCE

SGA Cache Entry Fixed Table

X$KDNST

Sequence Cache Statistics Fixed Table

X$KDXHS

Histogram structure Fixed Table

X$KDXST

Statistics collection Fixed Table

X$KGHLU

One-row summary of LRU statistics for the shared pool

X$KGLBODY

Derived from X$KGLOB (col kglhdnsp = 2)

X$KGLCLUSTER
Derived from X$KGLOB (col kglhdnsp = 5)

X$KGLINDEX
Derived from X$KGLOB (col kglhdnsp = 4)

X$KGLLC
Latch Clean-up state for library cache objects Fixed Table

X$KGLPN
Library cache pin Fixed Table

X$KGLTABLE
Derived from X$KGLOB (col kglhdnsp = 1)

X$KGLTR
Library Cache Translation Table entry Fixed Table

X$KGLTRIGGER
Derived from X$KGLOB (col kglhdnsp = 3)

X$KGLXS
Library Cache Access Table

X$KKMMD
Fixed table to look at what databases are mounted and their status

X$KKSBV
Cursor Cache Bind Variables

X$KSMSP
Each row represents a piece of memory in the shared pool

X$KSQDN
Global database name

X$KSQST
Enqueue statistics by type

X$KSUCF
Cost function for each Kernel Profile (join to X$KSUPL)

X$KSUPL
Resource Limit for each Kernel Profile

X$KSURU
Resource Usage for each Kernel Profile (join with X$KSUPL)

X$KSQST
Gets and waits for different types of enqueues

X$KTTVS
indicate tablespace that has valid save undo segments

X$KVII
Internal instance parameters set at instance initialization

X$KVIS
Oracle Data Block (size_t type) variables

X$KVIT
Instance internal flags, variables and parameters that can change during the life of an instance

X$KXFPCDS
Client Dequeue Statistics

X$KXFPCMS
Client Messages Statistics

X$KZDOS
Represent an os role as defined by the operating system

X$KZSRO
Security state Role: List of enabled roles

X$LE
Lock Element : each PCM lock that is used by the buffer cache (gc_db_locks)

X$MESSAGES
Displays all the different messages that can be sent to the Background processes

X$NLS_PARAMETERS
NLS database parameters