1. If you install the datbase in a box, and work
   directly from there ( without giving hostring) no
   listener is reuqired.
   like 
   Same goes with PERSONAL EDITIONS too.
2. But if u want u want to use a connection string...
   you have to use listener.
   same goes when u want to connect from cleint to server.
ex:

[b] local database -- without connect string [/b]
SQL> connect mag/mag 
Connected.
[b]local database with correct string only works[/b]
SQL> connect mag/mag@somestring
ERROR:
ORA-12154: TNS:could not resolve service name

Warning: You are no longer connected to ORACLE.
SQL> connect mag/mag@local
Connected.
SQL> connect dbadmin@biochip_chipdb2
Enter password: *********
Connected.

3. You have to create the connectstring in the
   Tnsanames.ora file.
4. LISTENER waits or listens for incoming connections
5. single listener may handle many databases.
   for better security, 1 listener per database would
   be recomended.

ex: this shows that single listner is listening 
for many databases

$ lsnrctl status

LSNRCTL for Solaris: Version 8.1.7.0.0 - Production on 25-OCT-2002 11:27:37

(c) Copyright 1998 Oracle Corporation.  All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Solaris: Version 8.1.7.0.0 - Production
Start Date                21-AUG-2002 11:51:55
Uptime                    64 days 23 hr. 35 min. 41 sec
Trace Level               off
Security                  OFF
SNMP                      OFF
Listener Parameter File   /u01/app/oracle/product/8.1.7/network/admin/listener.ora
Listener Log File         /u01/app/oracle/product/8.1.7/network/log/listener.log
Services Summary...
  PLSExtProc            has 1 service handler(s)
  abprod                has 1 service handler(s)
  abtest                has 1 service handler(s)
  chipdb2               has 1 service handler(s)
  ctoracle              has 1 service handler(s)
  dc            has 1 service handler(s)
  dmea          has 1 service handler(s)
  nihcns                has 1 service handler(s)
  ods           has 1 service handler(s)
  pmwr          has 1 service handler(s)
  pmwr1         has 1 service handler(s)
The command completed successfully

6 . to check whether listener is up from a client, use TNSPING ( just like ping)

Microsoft Windows 2000 [[Version 5.00.2195]]
(C) Copyright 1985-2000 Microsoft Corp.

C:>tnsping biochip_chipdb2

TNS Ping Utility for 32-bit Windows: Version 8.1.6.0.0 - Production on 25-OCT-2002 11:32:37

(c) Copyright 1997 Oracle Corporation.  All rights reserved.

Attempting to contact (ADDRESS=(PROTOCOL=TCP)(HOST=*****.mit.edu)(PORT=****))
OK (70 msec)

C:>