OracleOraHome81Agent - Oracle 
Intelligent Agent. Similar to SNMP. Used with OEM, among other things.
OracleOraHome81AppServerListener - Listener for Oracle Apps
OracleOraHome81ClientCache - Used for Oracle Names Server cache replication
OracleOraHome81CMAdmin - Connection Manager, to handle user connections in a large MTS configuration
OracleOraHome81CMan - Part of the Connection Manager
OracleOraHome81DataGatherer - Used for the Capacity Planner, Performance Pack, etc.
OracleOraHome81ManagementServer - The middle tier of the OEM configuration
OracleOraHome81PagingServer - paging server to send pages. Usually used with OEM
OracleOraHome81TNSListener - receives connections from users and forwards the users to the appropriate database (it does NOT pass SQL back and forth as schwertner stated)
OracleServiceSID - This is your database
OracleSNMPPeerEncapsulator - Allows the Intelligent Agent to communicate with the master SNMP process
OracleSNMPPeerMasterAgent - Allows the Intelligent Agent to communicate with the master SNMP process
OracleWebAssistant - I believe this is for WebDB
The most important ones are the database itself and TNSLISTENER. If you have apps running, you may need the AppsListener. If this node is managed by OEM, you will need the Intelligent Agent. If this node IS the OEM server, you will need the Management Server, and probably the Intelligent Agent.
If you are not using any option, you can set them to disabled.