This is a nice explanation of the Birads scoring system: Click Here for the article.
When I first started reading mammograms in the early 80's there was really no official standard vocabulary for describing what was seen on a mammogram. Different readers would use different terms and it became confusing when 2 people read the same study, had the same recommendation but completely different verbiage in the body of the report. Birads was developed to standardize the conclusions of the reports so that no matter what the report said above the impression, the Birads would clear it all up. Over the years the system was improved by providing radiologists a list of acceptable terms to use in reports (the Birads Lexicon) so that all reports would be comparable and understood by all concerned.