Wikipedia actually offers a good straightforward explanation: An integrated development environment (IDE) is a software application that provides comprehensive facilities to computer programmers for software development. An IDE normally consists of a source code editor, build automation tools and a debugger.
According to sources talked to for this article, useful tools and technologies found in IDEs can include:
• Code libraries
• A tag-based operating system
• Integrated runtime
• 64-bit software
• Compiled languages
• Tools synchronized across platforms and environments
>> Read Automation World's complete coverage, "How Development Environments Simplify PLC Programming".