Let me answer it like this for you.

The person who studies ethnographist goes out and picks ONE cultural group, say Yoruba, and studies their characteristics, behaviour, etc; then compiles that data.
Another ethnographist might chose the Efik culture for study, and after studying about them, compiles that data for record purposes.
Yet another ethnographist might decide to study the Kanuri and compile data on them.

Now, an ethnologist comes along, picks up the data on the Yoruba, picks up that of the Efik, and that of the Kanuri, compares them, and writes about their differences, similarities, etc.

Ethnography deals with the in-depth study of ONE cultural group, while ethnology concentrates on comparing data from different cultural groups.
Ethnology deals with the origin,distribution and description of a person's ethnic group while ethnography is the scientific study of the origin,behaviour, physical ,social and cultural growth of a human being.