Udacity is an online education provider that offers online courses, popularly known as “MOOCs” or Massive Open Online Courses, from universities and other entities around the world. Currently, Udacity boasts an active catalog of more than 200 online courses created by Udacity and these partner institutions. Some are self-paced, others are session-based or term-based.
Udacity courses consists of pre-recorded video lectures that you can watch on a weekly schedule or when it's convenient for you. They also have student discussion forums, project work/assignments, and online quizzes or exams.
Some single courses are free but you will need to pay for others. Nanodegrees are available on either a subscription basis or payable per degree or per term.
If you cannot afford the cost, Udacity offers a limited number of scholarships for particular Nanodegrees. Here is some information about Udacity scholarships: https://www.udacity.com/scholarships