When talking about Cisco presence products, we usually refer to BLF (Busy Lamp Field) and CUPS (Cisco Unified Presence Server).
BLF is a built-in feature of CUCM (CallManager). It allows you to see a person's phone status (off hook / on hook) on a speed dial button or in phone directory.
BLF: Pone 6001 is on-hook (idle)
BLF: Pone 6001 is off-hook (active)
For more information regarding CUCM presence, please see: http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/6_1_1/ccmfeat/fsprsnc.html
CUPS (Cisco Unified Presence Server) is a standalone products. It requires a separate server, it cannot coresident with CUCM. For supported server models, see: http://www.cisco.com/en/US/docs/voice_ip_comm/cups/7_0/english/compatibility/cupcompatibility.html
Usually, CUPS is used with a presence client - CUPC. The official name of CUPC is "Cisco Unified Personal Communicator". Some people also refer it as "Cisco Unified Presence Client".
CUPC can run on Windows and Mac OS. CUPC is a multi-function client. It can send IMs (like MSN Messenger). It can display your buddies' presence (on the phone, in a meeting, etc.). It can retrieve voicemail. It can control your desk phone. It can act as a soft phone as well. It support video calls. It can also be a outlook plugin. So you can "click-to-dial" from Outlook.
For more information regarding CUPC, please see: http://www.cisco.com/en/US/products/ps6844/index.html