What is a subquery?

Which part of the query/subquery is considered the inner query, and which part is considered the outer query?

Can a subquery always be done as a join? Why or why not?

When writing a query that will have a subquery, how do you determine which table/tables will go in the outer query?

Which predicate can usually be reformulated into a join?

When using operators, are many values acceptable from a result of a subquery?

What can you do to insure a working subquery?