1) Technology - Classic HDD is like a floppy disk from 80's, uses magnetics and mechanics technology. While SSD is only an electronic piece of a matrix of lot of capacitors.
2) Lifespan - besides the fact that SSD's capacitors have a limited life expectancy due to an early technology there is nothing else it can wear out, mechanical compounds of the HDD are more fragile from that point of view.
3) Speed - Especially on small files the difference is hughe, while reading the bit directly from a capacitor state is quite easy, moving an targeting of a header across the magnetic dish of the HDD consumes some time and is a limited by a mechanical factors. While reading big files that aren't spread on the whole dish is in a proper sequence is also easy, its a lot harder to read small pieces across the whole area because header have to move and acquire the track a lot more often.
Thats the basics.