I'm doing consulting work for an organization looking to expand their e-learning options. From my experience and readings about e-Learning, I believe it can take the following modalities, either in isolation or in combination.
The list is organized from the least to most interactive for students.
1) Access readings
Course instructors can post readings or links to readings on a public or secure (ie. log-in required) website. Readings can be in the form of webpages, PDFs, Word documents, PowerPoint presentations, etc.
2) Listen/watch asynchronous audio or video
Instructors can post to a website (or iTunes) audio or video files for students . These files can be either of formal lectures or additional informational resources (like readings) or lectures. Audio or video files can be in the form of YouTube, podcasts, Real audio, Quicktime, Flash, etc.
3) Email or upload
Students can be given the functionality to participate online by being able to:
a) send instructor questions via email or a web form
b) upload assignments (e.g, Word Docs, PowerPoints, spreadsheets, videos, photographs, etc.)
Chat can be in the form of text-only (e.g. instant messaging) or audio (eg. Skype). Video chats are also possible (for example, via webcams) but not yet technically seamless. Chats can include or be lead by the instructor or be students-only (for example, team discussions). They may be structured or a free-flowing Q&A.
5) Listen/watch synchronous audio or video
This could be a webcast of a lecture, but can also be a collective viewing of a presentation (e.g. over SlideShare) with a simultaneous teleconference or chat. Students can be given the option to interact with both the teacher or the student via email questions, Twitter, chat rooms, etc.
6) Surveys and polls
In e-learning, these seem to be mostly used to gauge the pulse of students on course topics or administrative issues, but can also be used for decision-making.
7) Interactive quizzes or tests
Students can view questions on the screen and provide their answers online. Options include assessing or grading their answers automatically in real-time or by instructors. This can be in the form of webpages, Flash, etc.
8) Educational online games/experiences
There are a huge variety of educational games, but they normally are animated with sound and allow the student to interact and receive feedback from the game.
9) Online discussions
Also called message boards, threaded posts, and forums, discussions appear to be the mainstay of e-learning. The technology gives the functionality for someone to start a specific topic (i.e. conversation thread) and then others can reply in a specific online space for that discussion. There are various options for how to structure these, for example conversation topics can either be assigned or open, the entire class participates or they can be team-based, optional participation or required, instructor participates or students only.
10) Online collaboration
Students work collectively on course assignments. Assignments can be in the form of an essay, presentation, paper, etc. There are many tools to enable group collaboration online, including wikis, Google Docs, or simply email. Usually the instructor facilitates and answers questions but doesn't actively participate in the collaboration.
11) Virtual classrooms
A private virtual reality space (for example Second Life) can be built to resemble a traditional classroom or any desired venue. Students and instructors create avatars (online representations of themselves) and gather online simultaneously in the virtual space. The instructor can then lead a traditional-style lecture or Q&A session or enact an entirely new, multimedia event.