The word jinx is slang for a curse and is used either as a synonym for any curse that manifests in extreme bad luck or is used to denote a specific type of curse, usually one that involves "bad words" or pointing.
The word 'jinx' originally meant a type of curse that is cast with what I refer to as "bad words that stick" as well as by pointing at a person. Examples of jinxes would be phrases such as, "You'll never amount to anything", "You can't do anything right", "No girl/guy would ever want someone like you", "You ain't worth a dime", "You'll never be successful", "You screw everything up", etc. When these phrases 'stick' to a person they work to destroy that person's self-confidence and ultimately bad luck will follow. Even yelling the word 'jinx' is enough to jinx someone, which is why many young baseball players will scream it out to their competition when a player is up to bat.
The second form of jinx involves pointing, either with one's fingers or an object. Also included in this is intimidation gestures, such as shooting a pretend gun or even rifle with one's fingers. Of course when doing this one is unconsciously sending ill-will, even possibly death-wishes, to the person. Now, with regard to pointing I'm not referring to the typical pointing where one may indicate directions to a lost person or one may use to count something. Instead, I am referring to pointing that takes place in heated argument. A skilled jinxer will incorporate both pointing and 'bad words'.
In modern times the word jinx has become quite popular and has even developed an additional meaning, such as discussing something before it officially happens. An example of this would be not talking about going to a job interview that went very well for fear that one will jinx one's self and not get the job. Additionally, some people use the word jinx as a synonym for any curse, especially if the curse manifests as bad luck.
In order to protect one's self from jinxes it is important to enforce one's will and confidence. Jinxes only work when they "stick". If one develops a good strong will and self-confidence then the likelihood of a jinx sticking is slim. Other ways one may protect themselves are by crossing one's fingers or by spitting. The latter derives from superstitions regarding the evil-eye and the former is used primarily when a person is discussing something that hasn't officially happened yet. If by chance someone does become jinxed, as in the jinx stuck to the person good, then cleansing or reversal rituals and spells will need to be performed to remove the jinx or reverse it back to the person who caused it.
Note: People can and do jinx themselves with negative 'self-talk'. If you find that you are constantly putting yourself down with negative self-talk, such as "I'll never lose weight", "I'm not good enough", "I don't deserve this-or-that", etc., then make sure to correct such thoughts and counter-act them with positive self-talk.