A cryptogram is a phrase or quote that has been encrypted by simple letter substitution. The phrase "This is a cryptogram" might turn into the encrypted phrase:
In this cryptogram you can see that throughout the phrase the letter "T" has been replaced by the letter "F, the letter "A" has been substituted with "U", and so on.
One final rule - a letter can never be substituted with itself.
You solve the cryptogram with a trial and error process, guessing the letters that have been replaced.
After you work on cryptograms for a while you learn to see patterns of common words - practice makes perfect!
Here are a few tips to get you going:
Have fun and good luck!