in a nut shell its 7 led's that share the same cathode (- side) then you multiply it by how many digits
so if you have a 4 digit display you will have 7 or 8(decimal point) pins going to led's and 4 cathodes
the magic trick is to display them 1 number at a time, quick enough that you cant see them blink on and off
doing this can be direct to arduino, or you can use some shift registers
or any other million and one ways to tackle the problem, each with their advantages and disadvantages ... for instance shift registers cant provide much current so your limited to a rather dim display, especially if each number is only on for 1/4 the time