Haswell is the codename for Intel's processor microarchitecture that serves as the successor to the Sandy Bridge and Ivy Bridge architectures. Like Ivy Bridge, Haswell utilizes a 22nm (nanometer) die shrink fabrication process, and it serves as the "tock" in Intel's alternating tick-tock model of releasing new processor families. The "tick" to Haswell, codenamed Broadwell, will be fabricated on a 14nm die shrink.
Processors utilizing Haswell were officially announced and released by Intel in June 2013 as the "4th Generation Intel Core Processor Family," with each Haswell processor bearing a 4-digit number with an initial digit of "4." This follows the pattern of Ivy Bridge processors bearing a number in the "3xxx" range and Sandy Bridge processors falling in the "2xxx" range.
The successors to Haswell and Broadwell will be Skylake and Skymont, and they're expected to use a 14nm die shrink and 10nm die shrink, respectively.