Jail. The public areas of the iPhone or iPod to which, by default, Apple allows read/write access via USB. In Unix terms, this refers to the /private/var/root/Media folder.
Jailbreak. The iPhone and iPod touch hacks that allow users to gain access to the entire Unix filesystem. In Unix terms, this refers to changing the root of the directory tree to /.
Activation. The process that allows you to move beyond any of the various screens that instruct you to connect your device to iTunes before it can be used. On the iPhone, you can only make emergency calls until your iPhone is activated.
Bricking. To render an iPhone or iPod touch inoperable. The 1.1.1 firmware update turned many iPhones into iBricks. Users could not reactivate their iPhone to get past their "Please connect to iTunes" screens. Although the phones could still be used for emergency calls, users were locked out from all normal iPhone operations.
File system. The way your iPhone or iPod touch uses its memory to store data and applications. The iPhone and iPod touch use two "disks": a smaller private file system that contains the operating system and a larger public one that contains your media (songs, videos, etc), preferences, and data.