Answer:
Just to add a little more detail
According to the Energentic Materials Research and Testing Center (EMRTC) in the courseware for Incident Response to Terrorist Bombing
"An IEDs has four basic components which can be recalled by the acronym PIES:"
P- Power Supply (electric or non-electric)
I- Initiator - the component that starts the explosive chain ie squibs, blasting caps, flash bulbs, exposed filament)
E - Explosive either high or low. Can be commercial such as ANFO, Military such as SEMTEX or Homemade exposive like HMTD or TATP
S-Switches (Two Catagories mechanical or electrical) Switches usually perform two functions, safety arming & detonating