General Background Processes.

Acronym	Process Name	Description
SMON	System Monitor Process	Performs recovery after instance failure and monitors temporary segments and extents.
PMON	Process Monitor	Recovers failed process resources. If Shared Server architecture is used, PMON monitors and restarts any failed dispatcher or server processes.
LGWR	Log Writer Process	Writes the log buffer out to the redo logs.
DBWn	Database Writer Process	Writes dirty buffers from the buffer cache to the data files. Possible processes include DBW0–DBW9 and DBWa–DBWz
CKPT	Checkpoint Process	Writes checkpoint information to control files and data file headers.
ARCn	Archiver Process	Writes filled redo logs to the archive log location(s). Possible processes include ARC0–ARC9 and ARCa–ARCt.
DIA0	Diagnostic Process	Responsible for hang detection and deadlock resolution. Triggers DIAG to perform diagnostic tasks.
CJQ0	Job Queue Coordinator Process	Spawns slave processes (Jnnn) to execute jobs in the queue
MMAN	Memory Manager Process	Serves as the SGA Memory Broker and coordinates the sizing of the memory components
MMNL	Manageability Monitor Lite Process	Performs frequent and lightweight manageability related tasks, such as session history, capture and metrics computation.
MMON	Manageability Monitor Process	Collects statistics for the Automatic Workload Repository.
RECO	Recoverer Process	Resolves failures involving distributed transactions.

Advanced Queuing Background Processes

Acronym	Process Name	Description
Qnnn	AQ Server Class Process	Processes messaged in the Streams AQ queue. Spawned by QMNC.
QMNC	AQ Coordinator Process	Monitors message queues. Spawns Qnnn Processes

Automatic Storage Management Background Processes

Acronym	Process Name	Description
ARBn	ASM Rebalance Process	Rebalances data extents across the ASM file systems. Possible processes are ARB0 – ARB9 and ARBA.
ASMB	ASM Background Process	Communicates with the Automatic Storage Management instance.
DSKM	Slave Diskmon Process	Acts as conduit between RDBMS and ASM instances and the Master Diskmon daemon to communicate I/O Fencing Information, I/O Resource Manager Plans, and Transaction Commit Cache information to SAGE storage. If no SAGE storage is used, the slave diskmon process will exit silently after startup of the instance.
GMON	ASM Disk Group Monitor Process	Maintains disk membership in ASM disk groups.
MARK	Mark AU for Resynchronization Coordinator Process	Marks ASM Allocation Units as stale following a missed write to an offline disk.
RBAL	ASM Rebalance Master Process	Coordinates rebalance activity for disk groups in an Automatic Storage Management instance. Performs global opens on Automatic Storage Management disks.
DRnn	ASM Disk Resynchronization Slave Process	Resynchronizes the contents of an offline disk. When a disk online SQL command is issued on a disk or disks that are offline, ASM spawns DRnn. Depending on the load, more than one slave may be spawned.

Data Guard Background Processes

Acronym	Process Name	Description
MRP0	Managed Standby Recovery Process	coordinates the read and apply process of redo in a physical standby database.
RFS	Remote File Server	is responsible for receiving the redo data, which is sent by the primary database to the standby database
LSP0	Logical Standby Coordinator Process	Assigns transactions to different Data Guard appliers and coordinates among them to ensure that dependencies between transactions are honored.
LSP1	Logical Standby Dictionary Build Process	Builds the data dictionary for the Logical Standby.
LSP2	Logical Standby Set Guard Process	Maintains Guard Standby information for the Logical Standby.
DMON	Data Guard Broker Monitor Process	runs on every instance in a Data Guard broker configuration. It communicates with local database and DMON processes of the remote databases. The broker-related requests and the monitoring information are transferred on this communication channel.
FSFP	Data Guard broker fast-start failover pinger process	is used for the management of fast-start failover status
RSM0	Data Guard Broker Worker Process	Used by the DMON process to manage and monitor the database.

Real Application Clusters Background Processes

Acronym	Process Name	Description
ACMS	Atomic Control File to Memory Server Process	Contributes to ensuring a distributed SGA memory update is either globally omitted on success or globally aborted in the event of a failure in an Oracle RAC environment.
GTXn	Global Transaction Process	Provides transparent support for XA global transactions in an Oracle RAC environment. The database auto tunes the number of these process based on the workload of XA global transactions. Possible processes are GTX0–GTX9 and GTXa– GTXj
LCK0	Instance Enqueue Background Process	Manage the global enqueued requests and the cross-instance broadcast. Handles all requests for resources other than data blocks.
LMD0	Global Enqueue Service Daemon 0 Process	Manages enqueue manager service requests for Global Cache Service enqueues to control access to global enqueues and resources. The LMD0 process also handles deadlock detection and remote enqueued requests. Remote resource requests are the requests originating from another instance.
LMON	Global Enqueue Service Monitor Process	Monitors the entire cluster to manage the global enqueues and the resources. Manages instance and process failures and the associated recovery for the Global Cache Service (GCS) and Global Enqueue Service (GES). In particular LMON handles the part of recovery associated with global resources. LMON-provided services are also known as cluster group services (CGS).
LMSn	Global Cache Service Process	Handles remote Global Cache Service (GCS) messages. The number of LMS processes varies depending on the amount of messaging traffic among nodes in the cluster.
PING	Interconnect Latency Measurement Process	Assesses the latencies associated with communications for each pair of instances. Every few seconds, the process in one instance (INSTANCE_NUMBER value) sends two messages to each instance (TARGET_INSTANCE value). One message has a size of 500 bytes and the other has a size of 8 KB. The message is received by the PING process on the target instance and is immediately acknowledged. The time for the round-trip is measured and collected.
RMSn	Oracle RAC Management Process	Performs manageability tasks for Oracle RAC, such as creation of RAC-related resources when new instances are added to the clusters.
RSMN	Remote Slave Monitor Process	Manages background slave process creation and communication on remote instances.

Source : Oracle Documentation