Rows in a rack configuration

A chassis is a computer case, typically which is populated with computer components, like a wiring harness, motherboard, disks, power supplies, etc. Although sometimes a chassis has slots for multiple motherboards. Usually you'd buy a server assembled, but you can buy parts (which hopefully match) and get a naked chassis.

A rack is a frame, like a closet on wheels (and feet) and maybe doors and sides. A rack can be two pole (like a network rack, without sides) or four pole. The "poles" are tall square brackets with supports to keep it rigid and holes for mounting hardware to hang multiple chassis in the rack. Network hardware and PDUs typically have "ears" to mount them into a two or four pole rack. Server chassis usually come with rails which attach to the four poles and then you slide the server into the rails.