a portal is a synonym for web site. DNN multiportal feature allows you to create a large number of portals using the same installation files, all portals will share code (including modules), database and host settings, but are separate apart from this and shall usually not interfere at all (there is a limitation: the same username cannot be used in multiple portals of your installation except with the same passwort).

The decision, whether to user a single installation for all portals or multiple portals with separate installations depend on your business model. a single installation is easier to handle and has less server requirements, but if a web site grows and needs dedicated resources, it is a lot of work to separate it. on a shared installation you cannot backup/restore single portals, only all of them.

Child portals are usually used for projects, divisions, ... that use the same domain address (with a folder name added) but a separate membership and management. In many cases it is also possible to use a subdomain (myproject.mydomain.com) with a parant portal instead of using a subportal (www.mydomain.com/myproject).

HTH