terminology - instance, user, tablespace

I have a bit problems in understanding the term instance, users and tablespace in Oracle or databases in general.

An instance is identified by a SID, say ORC0. I assume, in unix terminology it is a server process that acts on a certain set of data.

When create table statements are executed by a certain user, these tables are unique in this instance, right? It is not possible to run two different sets of tables (tablespace?) for the same user in an instance.

So when I want to have two different databases, say one with test data, the other with production data, I would have to create two instances.

Are the above considerations correct?

-- 
Chris Christoph P. U. Kukulies kuku_at_gil.physik.rwth-aachen.de