What is the SID (Service Identifier) in Oracle?

Oracle uses service (oracle service) for connecting instance database by looking to sid. You can find which service names (sid) are using in database services with this command easily.

Connection format: IP:LSTN_PORT/SID
e.g.
C:\Users\Alper> lsnrctl status

LSNRCTL for 32-bit Windows: Version 11.2.0.2.0 - Production on 11-EKI-2014 12:01:25

Copyright (c) 1991, 2010, Oracle.  All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC1)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for 32-bit Windows: Version 11.2.0.2.0 - Production
Start Date                11-EKI-2014 11:49:12
Uptime                    0 days 0 hr. 12 min. 16 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Default Service           XE
Listener Parameter File   C:\oraclexe\app\oracle\product\11.2.0\server\network\admin\listener.ora
Listener Log File         C:\oraclexe\app\oracle\diag\tnslsnr\Alper-PC\listener\alert\log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(PIPENAME=\\.\pipe\EXTPROC1ipc)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=Alper-PC)(PORT=1521)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=Alper-PC)(PORT=8080))(Presentation=HTTP)(Session=RAW))
Services Summary...
Service "CLRExtProc" has 1 instance(s).
  Instance "CLRExtProc", status UNKNOWN, has 1 handler(s) for this service...
Service "PLSExtProc" has 1 instance(s).
  Instance "PLSExtProc", status UNKNOWN, has 1 handler(s) for this service...
Service "XEXDB" has 1 instance(s).
  Instance "xe", status READY, has 1 handler(s) for this service...
Service "xe" has 1 instance(s).
  Instance "xe", status READY, has 1 handler(s) for this service...
The command completed successfully