A junkie most often refers to a drug addict or an individual who abuses psychoactive drugs, especially someone who is dependent on heroin. This is based on the observation in early 20th century that many of those who abused illegal drugs sold scrap metal (or junk) to support their habit. This term has now evolved to pertain to someone who enjoys a particular hobby to an extreme level.
For instance, a music junkie is someone who listens to music most of the time (or all the time), even in the toilet or while sleeping. They cannot seem to survive a day without listening to the radio or their playlist.