Digital marketing, inbound marketing, and content marketing all make up the same ecosystem, yet they're different. We'll first tackle brief definitions for each, and then explain the hierarchy between the three.
Digital marketing can be defined as marketing involving computers and computer technologies. More specifically, it's the use of online media or native applications (or a combination of the two) for the purpose of acquiring new leads - preferably on a daily basis. There are various forms of digital marketing, and all involve one or more of the following: Written/text-based content, images/graphics, video, or audio. Also, some are paid (eg: via an advertisement network or a performing, industry-related website) while others are free (eg: social network business page).
Quite simply, digital marketing is the new way of saying Internet marketing. But because powerful and influential applications can now be downloaded and hosted on mobile devices such as smartphones and tablets (let alone laptops and desktops), a different term needed to be created so-to-speak. That said, digital marketing is still primarily conducted using Web-based applications, and probably will be for a while, as most agencies are reluctant to change.
Inbound marketing, probably our favorite term, can be summed up as any type of marketing that feeds a personal or company website. This is usually done through online promotional tactics, but it can also include offline marketing strategies as well (eg: flyer distribution, print or digital banners in brick & mortar establishments, television/radio advertising, etc).
The beauty of inbound marketing lies in the control a company or website administrator has with regards to intellectual property in the form of Web content, brand image, and online presence in general. This, of course, is relative to other types of digital marketing, social media marketing immediately coming to mind as an example. Depending on their policies, a social network more often than not has the power to disable your content without notice. Could you imagine relying on a network of millions of fans & followers to build your business only to wake up one morning with your account being shut down? Yikes. You'd probably be able to appeal, but for some organizations, it's incredibly difficult to recover from such a loss, even if it only lasts for a minimal amount of time.
Regarding inbound marketing, if you have a website that's well-scaled and secure, generally speaking, you'll continue to experience uptime, and therefore, responsiveness by your visitors and customers. This allows you to tweak your story or offer whenever you'd like, and users have access to these changes in real-time. You're also perceived as more of an authority in your niche when separated from competitors in your field who may be scattering their message across various channels - avoiding putting all their egss into one basket, ironically at a cost of spreading themselves too thin. In fact, this kind of leverage opens the door to new income opportunities beyond selling products and services exclusively; you or your company can actually become a leader in your industry, teaching other professionals how to build their businesses both off and online.
Content marketing is a unique, slightly-controversial form of marketing. Strongly associated with organic search engine optimization (i.e. SEO), it has established itself as a way of telling a story along with informing interested users as opposed to generating targeted traffic by means of manipulating the major search engines. In other words, it's become more personal and less mechanical. Rather than pleasing robots, we can & should today use content marketing to serve the needs of our customers and followers. It's clearly a natural form of marketing, one that, unfortunately, hasn't been nor continues to be at the forefront of businesses. For this reason, we would argue that content marketing fills the gaps of a previously-shattered sales culture.
The technical definition of content marketing can be summed up as marketing made up of one or more of the following placed onto a Web page (or as a blog post such as this one): text/typography, images/photos/graphics/infographics, video, and audio/podcasts. Its greater purpose is to attract interested visitors (who eventually become buyers, subscribers, etc), however, a side-benefit is that it keeps users on your site longer... allowing for an engaged audience and, consequently, recognition as a thought-leader.
It's definitely not as clear-cut as most marketers and marketing agencies would like for it to be, but the hierarchy between digital marketing, inbound marketing, and content marketing are fairly straight-forward. Based on our explanations for each above, it's easy to see that the latter is a subset of the former. There are some contrasts (i.e. some gray areas), but none as defined or important to the Web professional as what we already discussed. Rather than being viewed as a puzzle similar to the entire website-building equation (involving Web hosting, domain name hosting, website security, content management, and more let alone the multiple marketing possibilities), the relationship between digital marketing, inbound marketing, and content marketing can be perceived as a line.