A trap error, which can be caused by either software or hardware, occurs when the processor "traps" an interrupt or exception and reports it to the operating system. An interrupt is defined as an event external to the processor that occurs randomly during system operation. The only time an exception can occur is when a special condition is detected by the processor during the execution of a set of instructions.
There are actually 256 possible trap errors, although we will only concern ourselves with the first 17 (0-16). These are all exceptions, except for trap 0002, which is a non-maskable interrupt (more on that later). Trap errors 17-31 are exceptions reserved by Intel and 32-255 are user defined maskable interrupts.