An extract is a dataset that has been programmatically extracted from a collection. There are two types of extract, raw and aggregated.
Raw extracts are just that. A raw data file that has been extracted from a collection. This is just plain ol' vanilla data with some filters applied. You can also specify test/control and pre/post flags to the data too.
Aggregated extracts are little more involved. These are extracts that have been summarized in some way. Rather all of the transactions for a few items being extracted into a file, an aggregated extract will summarize the data as it extracts it.
You can then specify the level of aggregation. At a day level, at a country-day level, at a country-store-day level, etc. This data can then go straight into a chart.