kernel: The kernel is the core or nucleus of an operating system. Basically, it provides a way for software and other parts of the operating system to communicate with a computer's hardware.
GNU: Part of Richard M. Stallman'sFree Software Foundation, it was founded in 1984 with the goal of creating a totally "free" operating system in which the source code was available to all who were interested.
This was both a technical and political aim. The first Linux kernel (see above) was complied by the GNU C programming language complier, gcc. Many of the first programs to be bound into the fledgling Linux operating system were from the GNU project. For this reason, Stallman insists that the operating system be called "GNU/Linux". The distribution Debian GNU/Linux follows this convention, for example, but most do not. Public use and custom has evolved as such so that most simply call it Linux.
Distribution: An operating system based around Linux Torvalds's Linux kernel. They come in all "shapes and sizes". Many are meant to be installed and be used by professionals on public Internet servers or on graphics rendering farms and other environments. Others are meant to be installed by the general public as an alternative to proprietary operating systems. Each distribution reflects either the needs of its target market, in the case of the commercial distributions or the wishes of its developers, as is the case with distributions created by individuals or non-profit organizations.
ISO images: This is a set of packages programs created to be eventually burned into (ie. copied to) CDs. A Linux distribution (see above) can be obtained by downloading the ISO images offered by companies and organizations and burning them to CDs.