In its fight against Google's Android, Apple want to avoid a repeat of what happened with Windows.
Everyone knows Microsoft ripped off the design of the Mac's operating system to create Windows. The little folders - the mouse - even famously the fonts used on its word processing program were inspired by Apple's design.
At the time, more than 25 years ago, Apple stood by and took it. The company didn't sue. In part because software wasn't as well protected by patents in the 80s. Today that's changed. Today software patents are common and Apple is using them aggressively to protect what it see as its innovative advantage in the smart phone space.
Patents suits have gotten a lot of criticism. Many folks in Silicon Valley believe that most software patents are overly broad and basically meaningless - but in the case of the Apple-Android patent war, is it possible this exactly what patent protection was intended to do.