Aida is a German cruise line - although it is part of the carnival Corp family of cruise lines.
Its ships appeal to German passengers with German being the primary language spoken onboard for shows, menus, etc.
The Aida ships were almost all built in Germany (unusual for Carnival Corp ships) at Meyer Werft. Like NCL, the tend to decorate the hulls of the ships, but they have no affiliation with NCL.
The ships are smallish compared to US cruise ships, but they are still priced in competition witrh mainstream lines like Costa and MSC which also appeal to Germans.