While legal definitions of stalking vary from one jurisdiction to another, a good working definition of stalking is a course of conduct directed at a specific person that would cause a reasonable person to feel fear. (Stalking Resource Center)
Stalking used to mean following someone around. The stereotype of a stalking was some creepy guy hanging out in the bushes spying on women. Stalking can still mean following someone, but technology has added some elements to the behavior.
Currently, stalking often means unwanted and/or numerous contacts from a person through texting, social media, dating apps, or phone calls.