Modern processors from both Intel and AMD have this feature which is essentially an automatic overclock. For example the Intel i7 3770K has a default speed of 3.5Ghz. Turbo Boost allows the CPU to increase it's speed based on workload. With all 4 cores under full load it goes to 3.6Ghz, 3 cores full load to 3.7Ghz, 2 cores at full load 3.8Ghz and only 1 core will allow the full Turbo speed of 3.9Ghz.
It manages cores like that to keep the heat down with a stock cooler. With aftermarket cooling you can overclock the chip higher.
Pretty good article explaining it.http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/53054-intel-i7-3770k-ivy-bridge-cpu-review-7.html