what the letter under the pennies date stand for?

[Sean U]

I also have some pennies that don't have a letter is it any valuable

 

[Husker41]

I believe that letter identifies the mint that produced the penny.
No, no extra value for no letter.

 

[E3_E3]

The letters mean what mint the penny was made at.

 

[just a man]

They are called mint marks.
The D is Denver.
The P is Philadelphia or no letter (plain) is Philly too.
The S is San Francisco.