I am using solr version 3.0.1, and I am about to change to solr 4.6.0.
Usually I just use solr without defining core (I think solr 3.0.1 doesn't have core yet).
And now I want to upgrade my solr to version 4.6.0, there is something new on it.
So i have 3 questions:

1. What exactly solr core is?
2. When i should use solr core?
3. Is it right that each solr core is like a table in a (relational) database? That is, can I save different type of data in different core?

Thanks in advance.