A creative advertising agency is responsible for creating and executing advertising campaigns for its clients. A creative agency is concerned with what messages to convey to the target audience, and how to communicate the message in a creative way that gets it noticed.
In creative agencies you'll see people take briefs from clients, then recommend the best way to answer that brief through the production of ads. They develop scripts for television commercials, then make those commercials. They develop billboard executions, radio ads, digital and social media content, or build digital experiences.
The process of developing ads can involve market research, developing concepts and messages, creating visuals and copy, and production.
The key thing that distinguishes a creative agency from a media agency is that the creative agency decides how best to communicate a message and produces the 'ad' that does so. The media agency is concerned with where to best place that ad so that it is seen by the right audience. They then negotiate with media companies (such as TVNZ, Stuff, or Google and Meta) to buy the space that the ad appears in.