What's the difference between Audience Builder and Audience Studio?

Audience Studio (AS) is a completely separate product. I believe it used to be called SFMC DMP. It allows you to ingest a ton of data from locations outside of SFMC and create audiences and segmentations for smarter marketing based off of that. These audiences are then used in Journey Builder, Email Sends, etc.

Audience Studio though is being outshone lately with the rise of Interaction Studio(IS). IS seems to be a much more robust and capable tool than AS ever was.comparing the two. It used to be Evergage before salesforce audience studio bought them.

Although both are fairly expensive, I have found many more feelings positive about Interaction Studio than Audience Studio. Most people I know that use AS wish they could transition off of it due to the cost and the many 'quirks' that happen with it.

That being said, depending on your context, use case and budget, both are viable options to choose from (or not choose).

Audience Builder is the 'section' inside SFMC that houses Contact Builder, Audience Studio, and other similar data oriented applications - similar to how 'Journey Builder' section houses both Automation Studio and Journey Builder applications.