VHDL is a common hardware description language, or HDL. It is used to describe all manners of digital systems in any complexity level, ranging from a gate-level description to a complex system
VHDL was developed by the Department of Defense. THis is evident in its name: VHSIC Hardware description language. VHSIC stands for Very High Speed Integrated Circuit. VHDL was developed from another DoD language, ADA, and sharse some features with ADA, most notably the fact that both have strongly typed variables and share syntax.
VHDL is NOT a programming language. It is a hardware description language. The main difference between the two is found in execution. VHDL is meant for devices, such as ASICs or FPGAs, whereas a programming language is meant for a microprocessor or microcontroller. The difference in execution is that VHDL executes in parallel, excepting for within sequential portions of the code such as processes, whereas programming languages execute sequentially.
There are two common uses for VHDL in industry: FPGAs, or the quite similar CPLDs, and ASICs. An FPGA is a field programmable gate array, which is essentially an array of digital logic devices which can be configured to implement a digital function. An ASIC is an aplication specific integrated circuit. It is a silicon chip fabricated by a semiconductor foundry to implement a function determined by the design of the ASIC. VHDL is used to describe the hardware used to implement functions in both technologies.
Benefits of using VHDL include portability, versatility, speed, and ease of development. VHDL is highly portable, owing to the fact that it is an IEEE standard. VHDL code may be ported from one manufacturer of FPGAs to another manufacturer of FPGAs or even to an ASIC. VHDL may also be able to describe functions at any arbitrary level of complexity, allowing for great versatility in what is described. VHDL, as a parallel execution HDL, allow the speed of the FPGA to be harnessed. Finally, owing to the ease of portability, widespread use, and aritrary level of complexity, it is simple to develop descriptions of functionality in VHDL.
A VHDL development environment is available free of charge from Xilinx, a company that designs FPGAs and CPLDs. This may be found here: http://www.xilinx.com/ise/logic_design_prod/webpack.htm
More information available on Wikipedia: http://en.wikipedia.org/wiki/VHDL