Part 1 is a checkbox and data filing covering assets under management, client types, custody arrangements, affiliations, and a disciplinary history section that is worth reading line by line. It is public through the adviser search database.
Part 2A is the brochure: strategies used, fees and how they are charged, other business activities, brokerage practices including soft dollars and whether the adviser directs trades to an affiliate, and conflicts of interest. Part 2B covers the individuals advising you. Part 3 is form-crs.
Annual updating amendments are required within 90 days of fiscal year end, and material changes must be delivered to clients. Comparing two consecutive years of a brochure is a cheap way to spot a change in strategy, fee structure or custody.
Related: registered-investment-adviser, investment-advisers-act-1940, form-crs, fiduciary-duty, finra-brokercheck