BCI provisions humans beings to control various devices with the help of brain waves. It is quite useful for the people who are totally paralysed from neuromuscular diseases such as spinal cord injury, brain stem stoke. BCI permits a muscular free channel for conveying the user intent into action which helps the people with motor disabilities to control their surroundings. Various non-invasive technologies like electroencephalogram (EEG), magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), etc. are available for capturing the brain signal. In this article, various non-invasive BCI devices are analysed and nature of signals captured by it are reported. We also explore the use of signals for diseases diagnosis, their features and availability of those devices in the market.