Whats the diffence between dohc and vtec?

i no it stands for varial timing electronic control or something like that. and dohc is dual over head cam. i no that cam shaft are wat open and close the valves. This wat i dont understand. i thought vtec was a replacement for cams...like it used a computer to open and close them. but i saw a car that said vtec-dohc. im kinda confused on exactly wat that mean. wat do car without vtec have instead? i have an 06 nissan sentra 1.8 so wat does my car have? also wat it meth injection?...is it something like nitrous?