Why does a typical PCB always has rounded tracks? What harm can a sharp edged PCB track can cause? Please explain! 
This is a great question, because the default answer is usually wrong for 99% of applications. The default answer is: to avoid reflections and other problems with high frequency signals.

The default answer assumes that you are dealing with very high frequency signals - signals with a wavelength that is small enough to fit a couple of times in your trace. When you regard such a signal as a wave, when it hits the end of a trace or a 90 degree corner, it gets reflected back and causes destructive interference with itself, attenuating the signal. 

However, almost all signals you will ever route through a PCB are either DC or - in terms of these kinds of problems - very low frequency. Even 1MHz is a very low frequency and you will not run into these kinds of problems. It's 100+ MHz that starts entering into routing problems. A great example of signals that benefit from clean layout in this respect are serial buses: PCIe, USB 2.0+, etc.

This does not mean that it's good practice to make sharp corners all over the place. There are a couple of reasons why you even want DC signals and basically all your routing to have nice 45 degree angles or rounded corners:


First of all, board area use. 90 degree angles or worse still: more than 90 degree sharp corners will always cause longer traces (higher impedance, more copper use) than traces that try to snake around obstacles. And often, your board size is limited, so you want to use as much area for the actual components, not the traces in between.
Cleanliness. A clean, nice-looking board layout is easier to optimize, transfer and  troubleshoot. 
Manufacturability. This is much less of a concern than in the past, but still something to consider if you plan on prototyping this on a hand-etched or milled PCB. Sharp corners tend to get loose when milled or get under-etched when using crude manual etching methods. Fluent lines are easier to produce. 


However, if you know what you are doing, don't hesitate to use sharp corners when you need them. As always: strict rules are for beginners and dumb people, once you know what you are doing you know when it's alright to deviate from the rules.
