Common Elements
Ruby and the Java programming language have a lot in common, beginning with the fact that they both follow some of the same object-oriented principles. Everything is an object in Ruby and--with a few small exceptions--that's true for Java as well. Both languages pass objects by reference and both languages are garbage collected (which, by the way, is not a sanitation issue, but a memory management system).
Comparing Code Execution
The real difference between the Ruby and Java programming languages lie in the method of executing code. Java code is first translated to a virtual machine language (that is, converted to a machine language for a simulated computer). Code executed on this virtual machine executes faster than Ruby's interpreted code.
With the exception of JRuby--which is itself implemented in Java--Ruby's current implementations are interpreted without translating to a virtual machine language first.
Differences in Syntax
Java's syntax is vastly different than Ruby's, too. The Java programming language follows a strict c-like syntax whereas Ruby's loose syntax frequently allows for things to be omitted. A good way to look at this difference is to compare the grammatical rules you'd have to follow when writing a term paper versus the freedom of language that text messaging provides.
Java uses a hefty set of mandatory keywords. In Ruby? Well, unless something other than the default is needed, some things are better left unsaid.
