What is email marketing and what are the benefits of it? Still its effective or not?

Email Marketing is using email to distribute your marketing messages.
Most popular is growing your own list, for example a newsletter, and sending emails to the list containing tips along with things subscribers might be interested in buying.
As well as your own list of tip-readers and potential customers, Email Marketing can also cover advertising on email newsletters or follow-up emails to your own customers.
While you could include these other wider definitions, most people when talking about Email Marketing are mainly using the term to describe building a list with a view to growing reader loyalty and making sales.