In ABAP instance we have one dispatcher and many work processes. Best way to learn work processes is DVEBMGS.
Dispatcher: It distributes all the request to different work processes.
Dialog: It fulfil all requests for the execution of dialog steps triggered by an active user. Every dispatcher requires at least two dialog work processes.
Update: It execute update requests. Similarly to spool work processes, you need at least one update work process per SAP system, and you can configure more than one per dispatcher.
Enqueue: It administers the lock table in the shared memory. The lock table contains the logical database locks of the ABAP runtime environment of the SAP system. Only one enqueue work process is needed for each system.
Background: It execute programs that run without interacting with the user. You need at least two background work processes for each SAP system. You can configure more than one background work process for each dispatcher.
Message Server handles communication between the distributed dispatchers within the AS ABAP, thereby enabling scalability of several parallel application servers. The message server is configured only once per SAP system.
Gateway enables communication between SAP systems, or between SAP systems and external application systems. There is one per dispatcher.
Spool: It passes sequential data flows on to printers. At least one spool work process is required for each SAP system. It is possible to configure more than one spool work process for each dispatcher.
ICM: Internet communication manager enables communication with SAP system using HTTP. Client sends the request to ICM and then ICM connects to ABAP+JAVA system. ICM decides which application server to call AS JAVA or AS ABAP depending on the nature of request. Only one ICM can be configured per application server.
AS JAVA Work Processes
Dispatcher distributes incoming requests to the server processes. The server process executes the Java applications. Every server process is multi-threaded and can thus process a large number of requests in parallel (in contrast to the ABAP work processes). For each dispatcher there is at least one server processes and there can be up to 16 server processes.
Java message service manages a list of Java dispatchers and server processes. It is responsible for the communication within the Java runtime environment.
Java enqueue service manages logical locks that are set by the executed Java application program in a server process.
Software Deployment Manager (SDM) is the standard tool used to install Java software components on the SAP Web AS Java. After SAP NetWeaver version 7.3 SDM is a part of SUM tool.