impedance and resistance
The difference between resistance and impedance is that impedance have reactive part beside resistive, which means that there is some phase shift between voltage and current. For AC systems reactive part is responsible for reflecting part of the received energy back to the source. This the major difference from DC when only ohmic or heat losses are present. But DC may be in the form of very fast rising and falling pulse or pulses. Actually such a pulse consists from DC and many AC signals. In this case there are a lot of additional losses.
When you design PCB you need to carefully consider what kind of signal will go through this particular trace. If it is pure DC you need to consider heat loss and possible coupling to other traces or elements. But if your trace will care any kind of AC signal you need to consider the signal properties and accommodate trace properties to match them. Usually RF signals goes through 50 Ohm lines and differential signals use 100 Ohm. Differential lines are more complicated, but they are more protected from noise and parasitic EM coupling. Any kind of traces or lines have resistance and according losses that must be accounted. However, AC impedance has much higher effect on board interconnections and often is major loss contributor. 50 Ohm was chosen primarily from feasibility and usability considerations. It is widely used value, but when it is necessary designer may chose other characteristic impedance for the trace. There is also important to divide or decouple DC power supply lines and AC interconnections, not only well visible forward path lines, but also practically invisible return path lines. This is not easy story, but if you want to be a good designer you need to learn it. The PCB design often underestimated, but it is probably not easier than all the circuits on the board. High frequency RF and high speed designs are considered as black magic (you may find the books with such words in the title). So, good PCB designer is often a magician.
Best regards,
RF-OM