Site Views and Areas are both concepts used for grouping parts of the Web Model. Both of them can have protection defined independently, so you could technically say that a group of users can access a particular Area while another group cannot.
The key difference between Site Views and Areas is actually that the former are completely disjoint: you can't link across Site Views using a modelled WebML link, while you can do it across Areas.
Is is common to have a Site View for the public ""front end"" site and another Site View for the ""bank end"" administartion area, since the two are usually completely separated.
As a rule of thumb I would create different Site Views when:
- there will not be links across them,
- they would provide substantially separated fronts to your application (like admin vs. public), and this requirement usually goes together with having two different user groups accessing them.
On the other end, go for Areas when:
- there will be links across them,
- you only need to group pages ""logically"", but not physically (like in the menu bar of a site).