Extractors are a type of validations that allows you to extract data from SmartTags.
Extraction process can occur before a SmartTag is processed or after a SmartTag is processed (Processing means execution of the SmartTags)
You can also provide customized JS code that allows you to extract many different elements from the whole page.
All the data that you extract are stored in a variable using the variable name.
As already mentioned there are pre and post actions:
Example of a pre-action could be of a link where you need to get the text and store that value to be used for further processing or further validations.
Post Action extractors is something that you could store in a setValue, and after setting the value you could validate what was set as the input for those accessors using Post extractions.