Bandwidth is the size of your pipe.
Download speed is how much water actually flows through it.
You can build an house with a 10″ diameter pipe, which would allow you to have tons of water, but then the water provider only sends you 2″ worth of water through it.
In terms of internet connection, the bandwidth is given by the physical infrastructure, which means the actual cables in the streets and your house. The download speed is dictated by the provider and your contract.
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