Compiler: a tool to convert high-level language source code (eg, 'C') to object code.
See http://www.8052.com/forum/read.phtml?id=15760 for the distinction between a Cross-Compiler and a Native Compiler;

Assembler: a tool to convert low-level, "assembly", language source code to object code.
There are also Cross- and Native Assemblers - see: http://www.8052.com/forum/read.phtml?id=18599

Linker: a tool to combine ("link") several object code files into one;

Locator: a tool to fix relocatable object file to specif addresses;

Linker-Locator: a tool combining the functions of both linker & locator - NB: most modern "Linkers" are, in fact, Linker-Locators;

Debugger: a tool for debugging your code;

Editor: a tool for creating & editing text files - including source code.

IDE: "Integrated Development Environment" - integrates all of the above (and usually more) into a single environment, usually with a GUI (Graphical User Interface)