Summarize this article in one sentence.
Stalking is a type of harassment, which is the act of making repeated or inappropriate contact with you that is unwanted and unreciprocated.  Stalking may take place in-person, with someone following you, spying on you, or approaching you at your home or workplace. The following may be signs of stalking receiving unwanted gifts, being followed, receiving unwanted mail or email messages, receiving unwanted or repetitive phone calls. Stalking can also occur online, in the form of cyber-stalking or cyber-bullying. These types of contact can be difficult to prosecute, but you may be able to avoid this harassment more easily by changing your online privacy settings or email address. Any instance of cyber-stalking that then transitions into in-person stalking should be considered very serious and should be reported immediately. Some types of stalkers are more dangerous than others, and knowing the type of stalker you are dealing with can help you notify the police appropriately and defend yourself if necessary.  Most stalkers are known as simple stalkers. These are individuals who you know that you may have had a romantic or friendly relationship in the past. The relationship ended for you, but not for the other person. Love obsession stalkers are individuals who you have never met (or very casual acquaintances) who latch onto you and think that they are in a relationship with you. People who stalk celebrities are in this category. Stalkers who have a psychotic fantasy about a relationship with their victims will often turn from unwanted attention to threats or intimidation. When this fails, they may escalate to violence. Sometimes the abuser in an abusive relationship or marriage becomes a stalker, following his ex and watching from afar, then moving closer, and eventually repeating or escalating violent attacks. This can be one of the most dangerous stalkers. A casual acquaintance who develops an obsession and drives by your residence occasionally or often may be ultimately harmless. An abusive ex-husband who has threatened you may try to kill you if you let your guard down.  If you are being stalked online, decide whether it is likely that the stalker has any information about your real-life whereabouts. Be sure to maintain a secure online presence and never reveal your home address or even your hometown on public pages. You should trust your instincts, be cognizant of the history of the person's behavior (if you are aware of it), and be realistic about the danger you're in. If you truly feel like you or your family members are in danger, you should seek help at your local police or sheriff's office or with a victim's services organization. If you think danger is imminent, call Emergency Services immediately. If you believe that you are being stalked, you should be extra observant of your surroundings. Notice anyone acting strangely or unknown vehicles in your neighborhood or near your workplace. Be sure to take notes about anything you observe that seems unusual.

Summary:
Know what qualifies as stalking. Determine the type of stalker you have. Sense how much danger you are in. Be observant.