An informed consent is a signed document on which the respondent explicitly declares to agree to give information about themselves for the objectives of the research.
Usually it is not allowed to gather information about a person. Especially when the information is delicate or very personal, people really can be harmed when this information becomes generally known.
In research a lot of information may be needed. To avoid problems respondents are asked to subscribe an informed consent. This document explicitly explains which information is gathered, what is done with this information and how the information will be handled. Respondents can be more sure that the delivered information is well protected and used for this research only.
In medical research it is quite normal to collect informed consents, in market research an informed consent is hardly ever used.
For youngsters who are not yet adults, the informed consent has to be subscribed by their parents or legal representatives.