What is the meaning of "parallel software" and what are the differences between "parallel software" and "regular software"?
What are its advantages and disadvantages?
Does writing "parallel software" require a specific hardware or programming language ?
Are you referring to Parallels which is the software used to run Windows on Macs? Link here: http://www.parallels.com/
parallel software can natively take advantage on multiple cores/cpus on a computer or sometimes across multiple computers. Examples include graphics rendering software and circuit design software.
Not so sure about disadvantages other than multi-processor aware software tends to be a CPU hog.
http://en.wikipedia.org/wiki/Thread_(computer_science)
In computer science, a thread of execution results from a fork of a computer program into two or more concurrently running tasks. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process. Multiple threads can exist within the same process and share resources such as memory, while different processes do not share these resources.
Most modern programming languages support multithreading in one way or another (even Javascript in the newest versions). :-)
Advantages and Disadvantages can depend on the task. If you have a lot of processing that you need to do, then multithreading can help you break that up into smaller units of work that each CPU can work on independently at the same time. However, multithreaded code will generally be more complex to write and maintain than single threaded code.
You can still write/run multithreaded code on a machine that has only one processor. Although there will only be one processor to execute the tasks, the operating system will ensure that they happen simultaneously by rapidly switching context and executing a few instructions for each thread at a time.
Some specialized hardware you may be familiar with which does parallel tasks is the GPU which can be found on most new computers. In this video, the Mythbusters demonstrate the difference between drawing on a single-threaded CPU, and a multi-threaded GPU:
http://www.youtube.com/watch?v=XtGf0HaW7x4&feature=player_embedded
Are the "parallel software" requires a specified hardware or programing language ?
Yes and Yes.
The first one is trivially easy. Most modern CPU's (Say anything newer than m6800) have hardware features that make it possible to do more than one thing at a time, though not necessarily both at the same time. For instance, when a timer interrupt goes off, a CPU could save what it's doing, and then start doing something else. Those tasks run concurrently.
Even without that, you could just get two machines with some sort of connection to each other (like a simple serial connection via a Null modem adapter) and they can both work on the same task in parallel.
Most new (not just modern but recent) CPU's have parallel computing resources built in. These multi-core CPU's can actually be working on two or more tasks at the same time, one task per core, and have special features that make it a bit more efficient for those tasks to cooperate.
The second one, requiring special software tools such as a parallel enabled language, is in some ways the hardest part of parallel computing. If you're the only person in the kitchen, it's pretty easy to cook a meal, by following each recipe from start to finish, one after the next, until all dishes are cooked. If you want to speed that up by adding more cooks, you have to be a bit more careful to not step on each other's toes.
The simplest way this is handled is by using a threading library that offers some tools so that multiple tasks can arrange to not clobber each other. This is not as easy as flagging a program as parallel and the system takes care of the rest, rather, you have to write each task to comminuicate with every other task at every place where there is the possibility of them interfering.