Microdata: survey or administrative data about an entity (person, family, establishment).
Summary statistics: aggregated counts of survey or administrative data.
Typically around a 2 year time lag from the time the data is collected to the time of release.
Longitudinal or panel study: same group of individuals is interviewed at intervals over a period of time.
Cross-sectional study: data from particular subjects are obtained only once.
Very little subnational data is available. Most subnational data is restricted when available. We do have a fair amount of state level macro data for the United States. City level data is often of a case study type or only available for very large cities.
Many micro-level datasets are restricted and it is not uncommon to wait a year before getting permission or denial to use the data. Each country has its own rules.
Aside from economic indicators and financial data, most data does not exist in electronic format. Historical data in electronic format prior to 1950 is rare. Most governmental links provide current data only.
The basic question to ask when looking for economic data is "who cares about what i am studying?" Unfortunately, the answer will often be no one. Ideally, look for an organization that is concerned with your research as part of its mission. Examples include the International Labor Organization focusing on labor research; the International Monetary Fund focusing on monetary and fiscal concerns; the World Bank focusing on development; and the World Health Organization focusing on health.
What is measured changes over time. Do not assume modern concepts were tracked in the past or even if they were that the definitions did not change.
What exists for one country may not exist for another country. Data is generally inconsistent across borders.
Documentation is typically in the native language.