the simplest sample of listener.ora and tnsnames.ora

 for listener.ora file 

LISTENER = (ADDRESS = (PROTOCOL = TCP)(HOST = ocm.localdomain)(PORT = 1521))

SID_LIST_LISTENER =(SID_DESC= (SID_NAME = clone)(ORACLE_HOME = /u01/oracle/product/database))

that's all

if the listener has muti address , then add address_list , and address is just one of the entry 

below is the normal type of an listener in admin/samples folder.

# LISTENER =

#  (ADDRESS_LIST=

#       (ADDRESS=(PROTOCOL=tcp)(HOST=localhost)(PORT=1521))

#       (ADDRESS=(PROTOCOL=ipc)(KEY=PNPKEY)))

# SID_LIST_<lsnr>

#   List of services the listener knows about and can connect

#   clients to.  There is no default.  See the Net8 Administrator's

#   Guide for more information.

#

# SID_LIST_LISTENER=

#   (SID_LIST=

#       (SID_DESC=

#                       #BEQUEATH CONFIG

#          (GLOBAL_DBNAME=salesdb.mycompany)

#          (SID_NAME=sid1)

#          (ORACLE_HOME=/private/app/oracle/product/8.0.3)

#                       #PRESPAWN CONFIG

#         (PRESPAWN_MAX=20)

#         (PRESPAWN_LIST=

#           (PRESPAWN_DESC=(PROTOCOL=tcp)(POOL_SIZE=2)(TIMEOUT=1))

#         )

#        )

#       )

for tnsnames.ora

ocm=(description=(address=(protocol=tcp)(port=1521)(host=ocm)) 

                     
4000
             (connect_data=(service_name=ocm))

                                   (failover_mode=(type=select)(mode=basic)(retries=10)(delay=10)

          )

for tnsnames.ora, key word  description can't be omit  and just like tns , key word address_list always can be omited.  for fail_over see the text upper in red

a concludsion is that all the key words which contain "LIST" can be omited.  DESCRIPTION_LIST , ADDRESS_LIST 

FAILOVER_MODE Subparameter	Description
BACKUP	Specify a different net service name for backup connections. A backup should be specified when using  preconnect  to pre-establish connections.
TYPE	Specify the type of failover. Three types of Oracle Net failover functionality are available by default to Oracle Call Interface (OCI) applications: session : Set to failover the session. If a user's connection is lost, a new session is automatically created for the user on the backup. This type of failover does not attempt to recover selects. select : Set to enable users with open cursors to continue fetching on them after failure. However, this mode involves overhead on the client side in normal select operations. none : This is the default. No failover functionality is used. This can also be explicitly specified to prevent failover from happening.
METHOD	Determines how fast failover occurs from the primary node to the backup node: basic : Set to establish connections at failover time. This option requires almost no work on the backup server until failover time. preconnect : Set to pre-established connections. This provides faster failover but requires that the backup instance be able to support all connections from every supported instance.
RETRIES	Specify the number of times to attempt to connect after a failover. If  DELAY  is specified,  RETRIES  defaults to five retry attempts. Note: If a callback function is registered, then this subparameter is ignored.
DELAY	Specify the amount of time in seconds to wait between connect attempts. If  RETRIES  is specified,  DELAY  defaults to one second. Note: If a callback function is registered, then this subparameter is ignored.