Put simply, a biosensor is a a device used to detect an analyte. The biosensor is comprised of a biological component with a detector.
The biological element that recognizes the analyte being studied can be an enzyme, antibody, tissue, microorganism, organelle or cellular receptor.
A biosensor also involves a detector or ‘physicochemical element.’ The detector can be optical, electrochemical, or piezoelectric. The detector transduces the signal (resulting from the interaction between analyte and biological element) into something that can be easily quantified.
A biosensor reader is also required to convey the information from the transducer in a user-friendly, interpretable manner.
To summarize: a biosensor is comprised of the biological element, detector (transducer) and the biosensor reader.
A typical example of a biosensor is the device which measures blood glucose. This biosensor uses the enzyme glucose oxidase to degrade glucose. However, this chemical reaction (catalyzed by glucose oxidase) generates a measurable current, because it is a reduction-oxidation reaction where electrons are transferred to reduce FAD to FADH2. By measuring the current, glucose levels can be inferred.
Some novel applications of biosensors are being pursued in the field of neuroscience. An example of this is the ion-channel switch biosensor. The BrainProtips Tumblr Blog is a great resource for news and information about the use of biosensors in neuroscience.