dynamic table/col names in plpgsql

Is there a way to use a variable as the name of a table or column in plpgsql?

This might be a simple question, but I can't find the answer in the
docs.  http://www.postgresql.org/docs/8.2/interactive/plpgsql-declarations.html
hints that there are data types that correspond to table/column, but I
don't see them listed here
http://www.postgresql.org/docs/8.2/interactive/datatype.html ...

Also, I assume that once I figure out the right datatypes, I can
assign to them from functions like substring()?

Thanks,
Gary