You can read more about digital marketing here : http://www.smedio.com/what-is-digital-marketing/
Online Digital Marketing is usually done and divided into big four channels.
SEO (SEARCH ENGINE OPTIMIZATION) - is used to increase your website visitors by obtaining high ranking placement result on search engine like Google, Bing and Yahoo
SOCIAL MEDIA MARKETING (SMM) - is a process use to obtain traffic from social networking sites like facebook, twitter and others
EMAIL MARKETING - is direct online marketing effort of sending email massively to current or potential customer/clients.
PPC/SEM (PAY PER CLICK/SEARCH ENGINE MARKETING) - is a popular types strategy using pay per click adverting from search engine and other advertising network or websites who offers paid advertisement to gain website visitors