In a single-blind study or experiment, the subjects don’t know if they are the experimental test subjects or members of the control group. But the tester knows. In short, the subjects are not told their identities in the setup.
In the double-blind setup, both the tester and the subjects are blinded: both are not told of the subjects’ identities in the setup.
In the triple-blind setup, the supervisor (or supervising committee), the tester and the subjects all are blinded: they are not told of the subjects’ identities in the setup.

The use of double-blind or triple-blind setups are useful avoiding demand characteristics and/or the placebo effect. We can have a better randomised, double-blind, placebo-controlled study this way. This is considered to be the superior model of clinical research since it eliminates outcomes that are produced due to placebo effect, as well as observer bias by the experimenter. The fact that the experimenter does not know which group received the placebo or the experimental drug means that the risk of conscious and unconscious observer bias is reduced, making the study more accurate.