The major difference between a pilot study and a pre-test is:
A pilot study is a complete study while a pre-test is not a complete study.
You do a pre-test when you have finished the design of your questionnaire. You invite about 10 people to try out the questionnaire and see whether there is problem in the questionnaire design.
On the other hand, a pilot study can be regarded as a light (mini) version of a study. Normally you do a small scale pilot study to confirm the study is workable before starting your expensive and time consuming data collection.