Hard Drive HDD/SSD
A HDD (hard drive) is a small object inside many computers containing a mass of moving parts that include a moving arm that reads the disk with all the computers saved documents, music, software and many other things. A SSD is a much more compact object with no moving parts. Its also alot faster but much more expensive.
An Optical Drive can be many different things like, a dvd player, a blue ray player or possiably a PS3 that contains a blue ray optical drive. All of these read information from a disc that plays out music, dvds and many other things.
A monitor is the 'screen' that displays all of the information that is sent to it.