Kickstart on the Amiga is the central part of the operating system, which is essential to running the Amiga. Normally it resides in ROM chips, but on the A1000 it was loaded from a floppy, and certain versions of the A3000 load it from harddisk.
Workbench is the name of the graphics work environment that resembles a bureau desk or the workbench of a craftsman. Here you can manipulate files with your mouse, or use tools, i.e. start programs. There is no artificial separation imposed between these two activities. The location of the files in the directory tree of their data volume is directly visible and not artificially hidden.
Datatypes are a new mechanism that was introduced with AmigaOS 3.0. It enables application programs to directly recognize what sort of data a certain file contains, be it graphics, music, text, or other types. Accordingly, there is now only one program needed to display (or play) all these types of files. Other programs then call on the Datatypes to automatically recognize and interpret these files. Datatypes are another element that shows the object oriented character of AmigaOS, as it's considered state-of-the-art for current operating systems.
IFF stands for "Interchange File Format" and defines a data format for storage and exchange of data between different programs. All Amiga applications support this standard, so that data like image files can be processed without problems by several programs after one another. IFF covers more than graphics, however, it also stores sound data, text, and configuration data.