Depending on the kind of drawing tablet you have you, you will have a very different body posture.
To quickly summarize:
With a pen tablet, your back will naturally have a more vertical posture
With a pen display, your will be hunched over a little or a lot.
With a standalone tablet, you will also be often hunched over.
When using the pen tablet on a desk...
The pen tablet will be laying typically flat on the desk - so it is parallel to the desk.
Your posture will be as follows
Your back will be vertical
Your head will be looking straight ahead at your monitor
Your eyes will be pointed straight ahead at your monitor
One hand will be holding the pen on the tablet
The other hand will usually be on the desk, often placed on or hear the keyboard to press shortcut keys
When using a pen display on a desk..
The pen display will mostly be at an angle to the desk and not flat because people:
Tilt the tablet using legs that come with the tablet
Tilt the tablet to varying degrees using a stand. Some tablets come with a stand and some are bought separately
Tilt the tablet using a monitor arm
Your posture will be as follows:
Your back will be leaning in toward the tablet
Your head will be angles down a bit towards at the tablet screen - how much you look down depends on on the tilt you have of the tablet.
Your eyes will be likely be looking down a little bit
One hand will be holding the pen on the tablet
The other hand will usually be on the desk, often placed on or hear the keyboard to press shortcut keys
Most people report that using a pen tablet gives them a better posture.
Even people who prefer to use pen displays typically have to be conscious of their posture and take breaks, stretch, etc.
Be aware MANY people find the posture of a pen display to be very difficult to deal with. I've seen many people buy a pen display and the return it and switch to a pen tablet because of this reason.
For some people the posture of a pen display is VERY uncomfortablet and can cause real pain.
Here is an example: