SSD-s are newer faster technology, they are built with electronic circuits. Consequently, they are faster. However , since they are built with this technology i think they are more prone to fail than HDD-s. More electronic circuits -> more heat -> transistors don't like heat, they change their region, or can even 'burn'.
HDD-s on the other hand, are older technology and today are mostly used for back-up. They use magnetic disks as their main storage unit. This/these magnetic disks store information which is accessed using a pin. They are slower than SSD-s. If you were to get a HDD now make sure to get a SATA.