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.