It's basically an open-source, interpreted, pure OO operating system. You can make changes to the system's code while it's running, any part of the system, and changes will take effect immediately.
The "interpreted" bit isn't really true anymore. Smalltalk has been bytecode-compiled since, uh, I think 1976, and the bytecodes themselves are JITted by the Pharo/Squeak virtual machine these days. They're usually only interpreted when you're using the debugger.
