Some important categories of SAP system parameters.

You can view parameters using RZ11 and change them using RZ10.
or use report RSPFPAR through SA38.
* ES/* : Related to extended memory settings
* INSTANCE* : Identify an instance or server
* ABAP/* : Related to program execution, including heap settings
* AUTH/* : Related to authorization
* DBS/<DB Type/* : Database specific parameters
* EM/* : General system related settings
* ENQUE/* : Settings for the enqueue WP (or standalone ENQ server)
* GW/* : SAP Gateway related settings
* ICF/* & ICM/* : Settings for the Internet control framework and ICM
* LOGIN/* : Controls the logon environment (such a multiple sessions etc)
* MS/* : Message server related settings
* RDISP/* : Controls the dispatcher & dispatcher controlled processes
* STAT/* : Statistics collector related parameters
* SCSA/* : Shared common system area parameters
* ZTTA/* : Extended memory related parameters