Theory:

A silent letter refers to those letters that are not pronounced when one is uttering a word. These letters exist in the word but does not serve the purpose of being spelt out together with the word. They may serve as a tool to emphasize or create an effect to the particular word.
There are considerable changes between the silent letters in British and American English, in both pronunciation and spelling levels. For example, in terms of pronunciation of the word herb, The 'h' is silent in British English. It is pronounced as 'erb'. The same thing applies to words like heart and holiday. Whereas in American English, it is pronounced with the 'h' making an impact. In terms of spelling, the Americans try to drop the letters that do not make any impact on the way it is pronounced. For example,
 
British English American English
ColourColor
FlavourFlavor
TheatreTheater
TravelledTraveled
 
Some silent letters in words include:
  1. Climb, Bomb, Debt, Doubt, Dumb, Numb - The letter 'B' is silent.
  2. Disciple, Muscle, Scene, Scissors - The letter 'C' is silent.
  3. Handsome, Sandwich, Wednesday, Porridge - The letter 'D' is silent.
  4. Align, Design, Gnarl, Foreign, Sign, Delight, Gnarled - The letter 'g' is silent.
  5. Honesty, Hour, What, Honest, Hours - The letter 'H' is silent.
  6. Knife, Knowledge, Kneel, Know, Knock, Knee - The letter 'K' is silent.
  7. Balm, Palm, Chalk, Walk, Half, Could - The letter 'L' is silent.
  8. Autumn, Column, Hymn - The letter 'N' is silent.
  9. Pneumonia, Psychiatrist, Pseudo, Receipt - The letter 'P' is silent.
  10. Aisle, Island - The letter 'S' is silent.
  11. Castle, Soften, Watch - The letter 'T' is silent.
  12. Guess, Guilt, Guitar - The letter 'U' is silent.
  13. Whole, Wrap, Wrinkle, Wrist, Wrestle,Wrong - The letter 'W' is silent.