How do hacks get detected

hey guyz

Obviously as u can see
i am a newbie xD

so i dont really understand how hacks get deteced...i read the sticky posts but not much of help

so can u guyz plz elaborate

x trap (ha.ck detector he is best) he works and some how detect code of ha.ck if code is strong and long they need long time to detect but if is small number of code ha.ck is detected for 1,2,3 days