In a traditional classroom, you see your students on a regular basis and communicate with them face-to-face. You watch them as they practice with the new material you're teaching, so you can get a feel for who understands and who doesn't. In an online classroom, you have to find other ways to communicate with your students because you will not ever see them in person.
The first thing you can do is provide your contact information to your students and set up virtual "office hours" when you can always be reached. Allow your students to communicate with you via email, instant messaging or phone conversations. Remember that in an online class many of your students are nontraditional, so they may have jobs or other classes they attend during the day and will need some evening or weekend hours during which they can contact you.
Once you have established ways for your students to contact you, check in with them regularly. Monitor your class site's discussion boards and forums to make sure all your students are actively participating. If a student isn't talking on the forums and isn't turning in assignments, send that student an email or call him to find out what is wrong or if there is any assistance you can offer.
Provide opportunities for your online students to interact with one another in small or large groups. Assign discussion board postings every week, where students have to respond to a prompt and then reply to their classmates' responses as well in order to earn full credit. Establish a rubric for grading this type of assignment to ensure that you receive substantial responses and not just basic agree/disagree responses.
Assign group projects during your course as well. Provide special group discussion board forums on your class's website so your students can collaborate in a discussion and post documents to share. Encourage students to use email and phone conversations to divide up the work and collaborate on the assignment before submitting it.
When you grade assignments in an online course, use a rubric so students know what to expect from the assignment before they begin it. Students will submit their work to you electronically, either via email or an upload to the course website. Grade these assignments using your rubric, then add your own comments to the assignments at the bottom and return them to the students so they understand why they earned the grade you gave.