A multilingual website is any website that offers content in more than one language. Examples of multilingual websites might include a Canadian business with an English and a French version, or a blog on Latin American soccer available in both Spanish and Portuguese.
A multi-regional website is one that explicitly targets users in different countries. Some sites are both multi-regional and multilingual (for example, a site might have different versions for the USA and for Canada, and both French and English versions of the Canadian content).
Each region/language will consist of a separate Website and related content within CRM.
Expanding a website to cover multiple countries and/or languages can be challenging. Because you have multiple versions of your site, any issues will be multiplied, so make sure you test your original site as thoroughly as possible before copying your original site to create subscriber site(s).
CMS Replication & Multilingual/Multi-Regional Portals