It's software. Don't be fooled by stock images of shiny coins emblazoned with modified Thai baht symbols. Bitcoin is a purely digital phenomenon, a set of protocols and processes.
It also is the most successful of hundreds of attempts to create virtual money through the use of cryptography, the science of making and breaking codes. Bitcoin has inspired hundreds of imitators, but it remains the largest cryptocurrency by market capitalization, a distinction it has held throughout its decade-plus history.