Includes types

Would like to know why we have 4 types of includes to select from: SSI File, SSI Virtual, PHP Include, Require.

I have used PHP Include only so far and it does what I need. Just curious.

While we are at it, is there a method to ‘cascade’ includes? So as to have an include, then another that combines the first include, then another include into one include?

In effect, like nesting DW template files.