EdX is an online education provider that offers online courses, popularly known as MOOCs or Massive Open Online Courses, from top universities and other institutions around the world. Currently, edX boasts a catalog of more than 2650 online courses created by these partners.
EdX courses consist of pre-recorded video lectures that you can watch on a weekly schedule or when it's convenient for you. They also have readings, student discussion forums, homework/assignments, and online quizzes or exams. There is an example course titled DemoX, which is designed to show new students how to take a course on edx.org: https://www.edx.org/course/demox-edx-demox-1-0
Generally speaking, edX courses are free to audit (i.e. watch videos and join discussions) for a limited period, but if you want unlimited access and earn a course Certificate, you will need to pay. EdX is the only leading MOOC provider that is both non-profit and open-source.
EdX also offers a number of micro-credentials: MicroMasters, Xseries, Professional Certificate, and Professional Education. Here is a brief summary of each edX micro-credential:
The MicroMasters program grants credit that counts towards a Masters degree, if the learner who earned the credential is accepted into the on-campus program. MicroMasters contains a series of masters level courses.
XSeries was the first ever credential launched by edX, but few new XSeries are being released. The number of XSeries Certificate programs is currently 40. XSeries consists of sequence of courses that help you gain a deep understanding of a particular subject.
Professional Education are essentially paid courses, designed for professionals already working in the subject field.
Professional Certificates are similar to the MicroMasters and Xseries. Unlike MicroMasters, they do not offer a path to credit. Compared to Xseries, they are supposedly more employer friendly.