Most royal families follow the principle of primogeniture which is that the first born legitimate child inherits the throne. So, Charles is next. As Charles' first born, William will then become King after Charles. After William, it will be little Prince George. One day, when Prince George marries and has children, his first born will be the next in line. As time passes, the younger siblings of Charles and William and, ultimately, little George will see their families become more and distant from the throne and fall further and further down in the line of succession.
ETA: As pointed out by
@doni, the above is true for most European royal families but royals from the Middle East or Asia might have different succession paths.