Digital inequality is defined by our textbook (Social Media: Enduring Principles by Ashlee Humphreys) as "the difference in access to social, cultural and material resources necessary to access, use, and interpret digital information and technologies." This inequality is largely based off of the differences in economic, social and cultural capital between users. Economic capital is, of course, money. Social capital consists of the connections with other people that could be used as resources. Finally, cultural capital is the standards that create affiliation between you and others that fit in the same class.
So how does these things equate to digital inequality? Certain classes have more access and association to each of these different forms of capital while others are lacking. As a result, social media is used differently by these classes; the lower classes use media out of necessity, whereas the upper classes use it for self-expression and creativity.