Ads Manager is a tool where advertisers can create ads, analyze ad performance, and create ad reports. Ads Manager can be used independently of Business Manager, but advertisers who have set up Business Manager normally access Ads Manager via their Business Manager interface. A general reference resource for Ads Manager is available here.
Business Manager is a group of tools that enables advertisers to access a large variety of advertising options and capabilities, as well as share access to different advertising assets. A general overview of Business Manager is available here. A downloadable PDF guide is available here, along with a getting started guide here.