A brick is a rectangular object usually reddish orange in colour and is designed to build houses. It has been a substitute for wood and metal. This is because whenever there is a housefire, wood assists in making the fire bigger because it acts as a fuel while metal melts, which makes the house collapse more quickly and hence the chances of escaping a housefire would be reduced.
A bricked PSP is a PSP thats is F#$%ed up. The system software is missing and/or corrupt. Bascially the PSP is as useless as a brick probably more useless, I dont think u can make a house with a bunch of PSPs!. Well maybe you could but the house would collaspe in seconds. You can have an expensive paperweight however!
Bricking occurs when while you were updating your psp to custom firmware, it fails to copy a flasher and perhaps even copy something totally different. This happens because there is incorrect files in the xxxflasher folder during the update. Usually this is when your .dxar file was created with the wrong updates and so your flasher misjudges the files and of course bricking occurs. The flasher on the other hand, sometimes does something wrong by itself even though the .dxar file was formatted correctly. This happens when the flasher has been very badly designed and has bugs in it. On the other hand is a very rare case because flashers improve when new custom firmwares are released. NDS can br bricked via brickers or by errors during Flashing the firmware
A Bricker is simply a program that bricks a PSP/NDS. Once one knows a bricked PSP is useless, it is easy to put together the clues to realize that a bricker corrupts the firmware/breaks the PSP. They are like Viruses.