I know what a shifter is but I could not find a definitive answer on what an ispec shifter is or how is it different from a regular shifter. For example the 2014 shimano deore groupset says it has ispec shifters and ispec ready brakes.
I-spec is a Shimano-proprietary mounting system for Shimano parts. Its supposed to make mounting and adjusting brake+shift levers faster and easier by putting them on one mounting clamp.
I couldn't find a blurb on what it does / how it operates on the Shimano website, but the equivalent for SRAM is SRAM Matchmaker, which from their website:
"The MatchMaker X clamp is more than just a clamp. It’s like having Martha Stewart tidy up your cockpit. Since each clamp accepts up to three controls, you can put everything from your XLoc suspension lockout, your SRAM shifters and AVID brake levers as well as your Reverb XLoc remote, all on two clamps. Short of genetically engineering longer thumbs, it’s the ideal way to put everything within easy reach. Just like Martha would."
If you're running a mix of SRAM MatchMaker and Shimano I-Spec components, you can use a Problem Solvers MisMatch adapter to mix them up.
To expand on Batman's answer, here is a good picture of an ispec-ready shifter attach to the brake lever's clamp.
Here is a traditional shifter using its own clamp (SL-M591) taken from my potato camera