In the computer world, when we talk about concurrency, we talk about a series of tasks that run simultaneously in a computer.
This simultaneity can be real if the computer has more than one processor or a multi-core processor, or apparent if the computer has only one core processor.
All modern operating systems allow the execution of concurrent tasks. You can read your e-mails while you listen to music and read the news in a web page.
We can say that this kind of concurrency is a process-level concurrency
But inside a process, we can also have various simultaneous tasks and concurrent tasks that run inside a process are called threads
Another concept related to concurrency is parallelism.
There are different definitions and relations with the concurrency concept.
While some authors talk about concurrency when you execute your application with multiple threads in a single-core processor, so simultaneously you can see when your program execution is apparent.
Also, you can talk about parallelism when you execute your application with multiple threads in a multi-core processor or in a computer with more than one processor.
Some publishers talk about concurrency when the threads of the application are executed without a predefined order, and talk about parallelism when you use various threads to simplify the solution of a problem,
where all these threads are executed in an ordered way.
This chapter presents a number of recipes that show how to perform basic operations with threads using the Java 7 API. You will see how to create and run threads in a Java program,
how to control their execution, and how to group some threads to manipulate them as a unit.
The following link discusses Daemon and non-daemon Threads