E-Mail Marketing is a systematic process with a fixed structure, in which you can deliver marketing messages to your targeted audience (subscribers). The main advantage of this channel is that it grants the biggest return on investment (ROI). It's great not only for sales but also to communicate with your customers. There are many types of e-mails in marketing: Newsletter, Autoresponder, Triggered, Cyclical, Transactional, Conformational.
In E-Mail Marketing, the most important thing is to build a subscribers list. You can do it on many ways - not only online but also offline. Also, you have to remember to always get from these people a acceptance for the purpose of marketing. It's always a good practice to offer something in return, for example - you can give 10% discount only for customers that join you subscribers list. It can be highly beneficial in the future.