The AHJ require permits for work above a given amount of work. Mostly this is a dollar cost of o job. Most places also require that new construction be permitted. This is used to notify the AHJ and zoning boards of what is being built in there areas. Also the tax man uses this information for assessing property tax.
When inspections are required the permit is the adminstrative method used to notify AHJs to look at the work for code compliance.
Usually only a licensed master/contractor can obtain or "pull" the permit. Electrical,plumbing,HVAC, and general permits are common types of permits.
In most places a home owner can pull his own permit for the work he does in the house in which he lives. Some places haave restrictions on this. Other places have no permits or inspectors at all.
The permit system allows the AHJ to see that new places are inspected and meet the current codes. This system has a lot of differences depending on the local ordinances and/or state laws.
When ever you move to a new area to do work, you should always find out about the rules in that location. While many systems are simular, the differences can be very costly to find out about after you did a job in an unacceptable way there.
