Process vs threads

The distinction between processes and threads is important.

• Process: A process runs independently and isolated of other processes. It cannot directly access shared data in other processes. The resources of the process are allocated to it via the operating system, e.g. memory and CPU time.
  
• Threads: threads are so called lightweight processes which have their own call stack but an access shared data. Every thread has its own memory cache. If a thread reads shared data it stores this data in its own memory cache. A thread can re-read the shared data, when this happens in Java will be explained in Java memory model part of this article.
  

Within a Java application you work with several threads to archive parallel processing or asynchronously behavior.