An on-screen keyboard is exactly what is sounds like, a keyboard that is on the screen or monitor of your computer. It is another way to type or access your computer. It can replace the keyboard hardware that came or is part of your current system. An on-screen keyboard can be used with any program that requires text. This includes the internet, TextEdit, Microsoft Word, Excel, etc.
If the individual cannot use the standard computer (the hardware) due to physical limitations, an on screen keyboard can eliminate this issue. For example, if the user is using a head mouse for alternative mouse skills, the on screen keyboard would allow that individual to type. If the individual is able to effectively use any mouse (mouth mover, trackball, joystick for example) but has significant issues with keyboard use, the on screen keyboard would be very important. Individuals that would benefit from on-screen keyboard use can vary from Parkinson's to people with a spinal cord injury.
In order to "press your button" or click on the desired letter, dwell click software or an ability switch can be used. Dependent on the keyboard that works best for the user there are various ways that the keyboard could be modified. This could include the color, size of the font, ability to dwell and the features associated with that.