A process is an instance of execution that runs on a processor. The process uses any resources that the Linux kernel can handle to complete its task.
All processes running on Linux operating system are managed by the task_struct structure, which is also called a process descriptor. A process descriptor contains all the information necessary for a single process to run such as process identification, attributes of the process, and resources which construct the process. If you know the structure of the process, you can understand what is important for process execution and performance.
Reference: IBM RedBooks – Linux Performance and Tuning Guidelines.