With 44% of all retail sales in Europe set to be offline sales that are influenced by the Web in 2018, it's not surprising that online display advertising spend in Europe will grow more than three times faster than total advertising spend over the next five years. Forrester's Online Display Advertising Forecast  shows that online display advertising will continue to cannibalize advertising spend via other channels.

Firms are increasingly using video and rich media to engage, entertain, and attract the attention of online users and to enrich their brand storytelling. The growth of mobile device adoption and usage is also changing the way that users consume content:

1.Video: Google recently stated that video had entered a fourth dimension, incorporating sight, sound, motion, and interactivity to win the hearts and minds of video viewers. With more than 100 hours of video uploaded to YouTube every minute, Google has created YouTubeNation to curate video content and grow video audiences, especially among those ages 18 to 30.

2.Rich media: In Q1 2013, rich media click-through rates in the UK were more than double standard banner ads. A study by DG MediaMind found that the most popular rich media ad formats were those that were most easily viewable, which included floating ads, wallpaper ads, and commercial breaks.

3.Smartphones and tablets: 40% of YouTube viewing occurs on smartphones and tablets. Forrester's Online Display Forecast predicts that 48% of online display ads will be viewed on smartphones and tablets by 2019, up from 19% today. And it's not just device adoption that is driving our forecast; according to Ofcom, mobile handset users in the UK spent 12% more time on their devices in 2013 than in 2012.

Going forward, firms using online display ads will need to continue to differentiate them from traditional media advertising — and consider how increasing trust in online advertising from a consumer, publisher, and advertiser perspective can help it compete with TV advertising.