Simran comes from the Sanskrit "simrana" which means- 'act of remembrance', 'to remember continuosly'
In Punjabi, we refer to "simran" as a practice where we are dhyia' ing the Ek Akaal purakh. When we use gurmantar or other mantar like har har or ram, then we are using the technique of japa yoga. This is "jaap"
Jaap comes under the umbrella of simran, but it is just one technique that's used to get started. Then comes shabad and meditating on shabad/naad, which is practice of nada yoga. Then we have rom rom, and following this; are some advanced techniques using nada, rom rom and surti to successfully leave/or enter, via the dasam. All of these come under the same umbrella of simran and us trying to "Si-Mar" (Mar= to die, si-mar is to die in the shabad by doing simrana)