What is the Difference Between a KNX Line Coupler and a Line Repeater?

At a product level they are exactly the same device, it is only when configured in ETS that they become different.

A Line Coupler links a line to an area or an area to a backbone area and has the filter table enabled to manage the amount of traffic on the bus. This can be opened in the parameters if it is a small system or for troubleshooting. For a line coupler the address would be x.x.0 and for an area coupler it would be x.0.0. Only line couplers can have these addresses.

A line repeater has the filter table turn off and is used to join up to 3 additional line segments to the first segment. A line repeater can have any physical address although it is common to give them their correct topological address of x.x.64, x.x.128, x.x.192 or group them at the beginning of the line x.x.1, x.x.2, x.x.3.

Remember that every line or segment needs its own power supply.