There can be multiple instances running in a SQL server. There are two
types of instances: default instances and named instances.
A named instance simply means that you define a name for an instance
during installation and that you can access the instance using this
name. A default instance is the one that is default. There can be only
one default instance but multiple names instances. When thee are
multiple instance then it leads to multiple services running which
affects the system performance. Each instance has its own SQL server and
SQL server agents services. For default instance the name of these
services are MSSQLSERVER and SQLSERVERAGENT and for a named instance it
is MSSQL$instancename and SQLAgent$instancename.