If your company is business-to-business (B2B), your digital marketing efforts are likely to be centered around online lead generation, with the end goal being for someone to speak to a salesperson. For that reason, the role of your marketing strategy is to attract and convert the highest quality leads for your salespeople via your website and supporting digital channels.
In some contexts, the term SEM is used exclusively to mean pay per click advertising, particularly in the commercial advertising and marketing communities which have a vested interest in this narrow definition. Such usage excludes the wider search marketing community that is engaged in other forms of SEM such as search engine optimization and search retargeting.
Digital marketing is defined by the use of numerous digital tactics and channels to connect with customers where they spend much of their time: online. From the website itself to a business's online branding assets -- digital advertising, email marketing, online brochures, and beyond -- there's a spectrum of tactics that fall under the umbrella of "digital marketing."