From MSDN Library:

ABOVE_NORMAL_PRIORITY_CLASS 0x00008000
Process that has priority above NORMAL_PRIORITY_CLASS but below HIGH_PRIORITY_CLASS. Windows NT and Windows Me/98/95: This value is not supported.

BELOW_NORMAL_PRIORITY_CLASS 0x00004000
Process that has priority above IDLE_PRIORITY_CLASS but below NORMAL_PRIORITY_CLASS. Windows NT and Windows Me/98/95: This value is not supported.

HIGH_PRIORITY_CLASS 0x00000080
Process that performs time-critical tasks that must be executed immediately. The threads of the process preempt the threads of normal or idle priority class processes. An example is the Task List, which must respond quickly when called by the user, regardless of the load on the operating system. Use extreme care when using the high-priority class, because a high-priority class application can use nearly all available CPU time.

IDLE_PRIORITY_CLASS 0x00000040
Process whose threads run only when the system is idle. The threads of the process are preempted by the threads of any process running in a higher priority class. An example is a screen saver. The idle-priority class is inherited by child processes.

NORMAL_PRIORITY_CLASS 0x00000020
Process with no special scheduling needs.

REALTIME_PRIORITY_CLASS 0x00000100
Process that has the highest possible priority. The threads of the process preempt the threads of all other processes, including operating system processes performing important tasks. For example, a real-time process that executes for more than a very brief interval can cause disk caches not to flush or cause the mouse to be unresponsive.

Every thread has a base priority level determined by the thread's priority value and the priority class of its process. The system uses the base priority level of all executable threads to determine which thread gets the next slice of CPU time. The SetThreadPriority function enables setting the base priority level of a thread relative to the priority class of its process.

and for CREATE_NO_WINDOW

The process is a console application that is run without a console window. This flag is valid only when starting a console application.