How does a modern switch differ from an old hub?

A hub is a dumb physical-layer repeater: any bits arriving on one port are blasted out all other ports, so every device shares one collision domain, only one device can transmit at a time without colliding, and the medium ran half-duplex with CSMA/CD (carrier sense multiple access with collision detection) to arbitrate. A switch is a layer-2 device that learns which MAC address is reachable on which port, by recording the source MAC of frames it sees, and then forwards each unicast frame only to the port where the destination MAC lives. That gives each port its own collision domain, so devices run full-duplex with no collisions at all, and aggregate bandwidth scales because multiple conversations happen simultaneously on different ports. Broadcasts and unknown-destination frames still go to all ports (the broadcast domain), and switches build a MAC address table that ages out stale entries. The upshot is that switched full-duplex Ethernet eliminated the collision-and-backoff regime that limited shared hubs, which is why CSMA/CD is essentially historical for wired Ethernet today.