From Snoboy's link, regarding the X:
When the Wright brothers first took to the air in 1903, there was no need for coding airports since an airport was literally any convenient field with a strong wind. However, the National Weather Service did tabulate data from cities around the country using a two-letter identification system. Early airlines simply copied this system, but as airline service exploded in the 1930's, towns without weather station codes needed identification. A bureaucrat had a brainstorm, and the three-letter system was born, giving a seemingly endless 17,576 different combinations. To ease the transition, existing airports placed an X after the weather station code. The Los Angeles tag became LAX.