Summer time and daylight saving time

I happen to find that people who live in the U.S. call "summer time" "daylight saving time".

When I sent an email to my colleague, it seemed that he could not understand what "summer time" means. After few times of email exchange, we finally found out that "daylight saving time", means dst, and equivalent to "summer time".

It is interesting that there are two names "S.T" and "DST", and dst is more frequently used in U.S.