"In public speaking, back-channeling is the practice of electronically passing notes among some or all of the audience/students during the lecture. This practice may be sanctioned as useful for speakers who are attempting to dynamically modify their presentations based on immediate feedback from the audience. It may be unsanctioned and discouraged as distracting for the presenter. Meebo and Twitter are among the common back channeling devices."
Although we wouldn't recommend you do this all the time, back channelling can be a very handy way of engaging with an audience or getting immediate feedback on your talk.
However, if you want to set up your own back-channel, perhaps for a group of students in a single class and you don't want the world to be able to see the things your students are typing then you may wish to consider making use of some of these back-channelling solutions: