The earliest distance education courses may date back to the early 18th century in Europe. One of the earliest examples was from a 1728 advertisement in the Boston Gazette for "Caleb Phillips, Teacher of the new method of Short Hand," who sought students who wanted to learn through weekly mailed lessons.
The first distance education course in the modern sense was provided by Sir Isaac Pitman in the 1840s, who taught a system of shorthand by mailing texts transcribed into shorthand on postcards and receiving transcriptions from his students in return for correction - the element of student feedback was a crucial innovation of Pitman's system. This scheme was made possible by the introduction of uniform postage rates across England from 1840.
The very rapid spread of radio in the United States in the 1930s led to proposals to use it for distance education. By 1938, at least 200 city school systems, 25 state boards of education, and many colleges and universities broadcast educational programs for the public schools.
In the USA the University of Wisconsin-Madison developed a series of approaches to support off-campus learners in the 1960s and was an influence on the methods adopted by The Open University, established in the UK in 1969 as an entirely distance learning public university.
more on the Open University at wikipedia.org
By the mid-1990s a number of universities around the world had begun to support remote students through online bulletin board style forums and email systems. It was also during the 1990s that the World Wide Web begun to be established, offering an easy way to link resources in different locations through the use of hypertext markup language (html). This is perhaps the key point at which distance learning began to transition to online learning.
While the 1990s saw the rise of primarily asynchronous learning technologies, such as bulletin-board style forums and videos and text materials which could be studied at the student's own pace, the new millennium brought an increasing focus on synchronous communication with the rise of live streaming video, video and audio conferencing on the web and other technologies such as white-boarding and screen-sharing tools.
For more details read the Wikipedia article on Distance Education