STARTUP (entered in SQL*Plus) creates an Oracle instance.
When an instance is created, Oracle requires either an init.ora or an spfile.
The instance is terminated with a shutdown.

startup nomount

Only the instance is started. That is the SGA is allocated and the background processes are started.
Neither controlfiles are mounted nor database opened.

startup mount

Instance is started and controlfiles mounted, but database not opened.

startup

Instance is started and controlfiles mounted and database opened.

startup restrict

startup restrict is similar to startup, however, Oracle then allows only users with restricted privileges to access the database.

Logging

Oracle records startups of the instance in the alert.log file.

Links

ORA-01031 when starting an instance.