While all advertising agencies are by nature creative, when an agency says that they do creative advertising, they are implying that they employ a creative strategy and as a rule, take a more creative approach to advertising.
Some agencies feel that advertising has a set form and that it is best to create their advertising within that form, while others feel that in doing so, they are taking a risk that the advertising won't be memorable or effective. Creative advertising seeks to gain an edge on the competition and stand out by using a novel approach to communicate a brand's message.
The Creative Director
The person within an advertising agency who ultimately determines the level of creativity is the creative director. They oversee all creative work to ensure that the branding and advertising fits the image they wish to promote for their company or product. Creative directors interpret a client's communications strategy and then develop creative approaches that align with that strategy, so they are ultimately responsible for the quality of the final creative work.
When deciding how creative you want your company's advertising to be, it is best to understand what will work for your customers. Effective advertising sell products whether it is considered creative or not, which is the ultimate goal of all advertising.