What types of things in letters/postcards did the army censor during WWI?

The main thing that censors were looking for in letters
and postcards was information that would compromise what was called "operational
secrecy."  In other words, the censors did not want soldiers telling anything that would
allow the enemy to get any information about what units were stationed where or what
they might be planning to do.

As you can see in href="http://www.u.arizona.edu/~rstaley/wwlettr1.htm">this link, soldiers
were aware that their letters would be censored and they would not be able to tell such
basic things as where they were stationed.  If they enemy found out such information,
they might be able to piece together what units were in what places and, thereby, what
areas had strong defenses and which did not.  If a soldier wrote "we are in (place X)
and are about to move out" the enemy might be able to infer that an attack was planned
somewhere near Place X.

Basically, the censors were just
looking to delete anything that might be able to give a clue to the enemy about troop
locations and movements.