Digital marketing is primarily defined as the marketing of a product/service to a consumer using a digital medium.
The marketing effort can take many forms and is not necessarily limited to just the internet, digital marketing can take place via a computer or mobile where platforms like social media or email are used. Many start-ups consider the use of digital marketing due to the benefit of its large reach and its often little to no cost approach.
However, many start-ups can find themselves trying to use too many platforms instead of understanding where their audience lies and what platforms they are most likely to use. Photographers might choose more visual based social media such as Pinterest and Instagram, whereas online retailers may be more inclined to use Twitter for a more customer support based service.
One key tip would be to focus on a single form of digital marketing that you already have some experience with before expanding and trying others. Otherwise you may find yourself trying to manage too many different streams and social media accounts to make your marketing drive effective. Be successful and limit marketing avenues or take advantage of tools such as Hootsuite (link) which allows you to post to multiple social media accounts at the same time.