ColdFusion is a server-side rapid application development platform, implementing the dynamic general purpose CFML programming language
Railo is an Open Source (LGPL) rapid application development platform implementing the general-purpose CFML language, compatible with Adobe ColdFusion's implementation but with various extensions/updates. Don't forget to also use the `cfml` tag for any Railo questions involving the CFML language or any general CFML concepts.
There are many other small differences railo is generally stricter about syntax and semantics than adobe coldfusion and often those decisions are driven by performance concerns in that compatibility with adobe coldfusion would make railo slower
What could break when migrating from Adobe ColdFusion to an alternative CFML engine?