There is no any concept "ORACLE SERVER" but sometimes server where is the installed ORACLE DATABASE or other ORACLE APPLICATIONS called "ORACLE SERVER", sometimes server where is the installed ORACLE DATABASE people call as "DATABASE SERVER"
ORACLE DATABASE is software which is managed DATABASE.
Good question. There is something called Oracle Server. you can find the term in Oracle Documentation as well as the Oracle Education materials. We can define it as the combination of Oracle Database + Oracle Instance. As you would know already, Oracle Database is nothing but the physical files : CRD where as Oracle Instance is a combination of Memory Structures and Background processes...
Let me know if you need to know anything else..