First of all, as all nouns ending on -ung "Dienstleistung" is feminine: die Dienstleistung, die Dienstleistungen. "Dienstleistung" refers to a concrete service, e.g. serving meals or cutting people's hair. "Dienst", which is masculine, however refers to a more official context, where e.g. public servants "Dienst haben" or e.g. physicians and nurses in a hospital have also "Dienst" which can be seen on a "Dienstplan" - duty-roster. In a McDonalds restaurant you can have both: the person receiving your order does a "Dienstleistung" but on the same time she/he has "Dienst", as e.g. she/he doesn't work the whole day.