There are two main systems for metadata description (schemas) - specialised and generalised.
Specialised schemas require more time and expertise, but results in a much better description of the data. If your data matches a developed schema, the use of that schema will result in the best metadata for your data.
Generalised schemas require limited time and expertise, but the description may be inadequate. They include what are considered the essential elements to describe any data
Geospatial metadata (ISO 19115) - defines how to describe geographical information and associated services, including contents, spatial-temporal purchases, data quality, access and rights to use. The standard defines more than 400 metadata elements, and 20 core elements. An examples of an implementation of ISO19115 is Global Change Master Directory.
Sample Dublin Core metadata for an image
Below is an example of Dublin Core metadata for this photograph of Howard Walter Florey. Generic examples for the Dublin Core metadata schema are available from the Dublin Core website.
Title = Howard Walter Florey
Author/Creator = Not known
Subject and Keywords = The Australian National University photographs
Subject and Keywords = Nobel Laureates
Description = This photograph was taken at the 1948 Easter Conference, which considered the early planning of the Australian National University, at the Institute of Anatomy building. A keen amateur photographer, Florey carries his camera over his shoulder.
Publisher = The Australian National University Digital Collections