There are a lot of differences between xDSL and Fiber.
xDSL uses copper wires, which can be found in most households in Denmark. xDSL speeds vary, and the download rates are often higher than the upload.
Fiber is a newer technology, where download and upload runs synchronous, which means that the download and upload rates can be the same, for example 100/100 Mbit.
Fiber connections are more stabile, and typically support much higher speeds than xDSL connections.