Thread:

Creating a Thread is far slower than using Thread-pool.
You can change the priority of a thread.
The max number of threads in a process related to resources.
Thread is at the OS level and controlled by OS.
Using Thread is a better option when the task is relatively long-running
Thread-Pool:

Running a Thread on thread-pool is far faster than directly creating a Thread.
You can not change the priority of a thread run based on Thread-pool.
There is only one Thread-pool per process.
The Thread-pool is managed by CLR.
The Thread-pool is useful for short-lived operation.
The numbers of Threads in Thread-pool is related to the application load.
TPL tasks run based on Thread-pool