Haha good question.The 'speed' you buy from your ISP or cable company is the bandwidth from the local exchange/depot to your address.
It`s the width of that 'pipe' you`re paying for.Measured in Megabits per second (Mbit/s).
The speed you measure when using one of those internet speed tests (speedtest.net) is the actual speed you get from a server on the wider internet to your computer.
That will be affected by distance and general web conditions between you and there.