Demographics are statistical information about human populations (groups of people). The fields of sociology, demography, marketing, business, and others use the information that people who study demographics get. Some demographics that many demographers study are gender, race, jobs, and location (where people live).
Demographic trends are changes in those areas. For example, if a city was half women and half men in 1940 and was more than half women in 1980, and the number of women kept going up, that would be a demographic trend.