Perforating
Production string and cement seales off pay zone, perforations must be made in order for the hydrocarbons to flow into the wellbore. Perforations are tunnels that are made through the casing and cement and extend some distance into the formation.

Perforating Guns
1- Casing gun
Allow perforation through  casing (Gun diameter is large) 
2- Expendable gun
A perforating gun assembly that disintegrates upon firing, creating finer debris. 
It is used  in through-tubing applications
3- Retrievable gun
It is retrievable from the wellbore after firing.  It generate minimal debris