Integrated Heat Spreader
An integrated heat spreader (or IHS for short) is a small cap made of metal that sits on top of the CPU die. It protects the CPU die from damage when installing a CPU cooler.
For a few years both AMD and Intel started manufacturing CPU's without IHS. The basic idea was that without the IHS, the CPU cooler would make direct contact with the die, resulting in improved heat transfer. Intel began this practice with the introduction of it's SECC2 package for it's Slot 1 CPU's and introduced IHS-less CPU's for Socket 370 at the introduction of Coppermine. AMD followed when it introduced it's Socket A CPU's.
Even though it did aid in cooling the CPU, it does come with a few disadvantages. The main disadvantage is that when installing a CPU cooler, there is a risk of cracking the die, killing the CPU in the process. This is especially true when someone tries to install the CPU cooler backwards.
Intel introduced the IHS with it's later Coppermine CPU's while AMD kept producing IHS-less CPU's all the way through it's Athlon and Athlon XP time periods.
What is also important to note about IHS's is that with an IHS, the CPU will be taller. This is important when trying to mount a CPU cooler that was designed for CPU's without IHS on top of a CPU that has an IHS. As the CPU clip is slightly shorter, it will need much more force before one can bend it enough to make it fit over the socket tabs. This way, enough force is applied to actually break (one of) the tabs and possibly scratching the motherboard.
On earlier CPU's with an IHS, the IHS can be removed, though it is a risky little operation. On later processors this was made even more difficult as CPU manufacturers started gluing the IHS's right to the CPU die, making it nearly impossible to remove the IHS anymore.