Magnet allows you to start a keyword search when you already have a preliminary keyword in mind, and want to find related keywords. It simply requires a starting keyword to trigger a broad search for related and relevant keywords.
Cerebro is a competitor keyword research tool. It allows you to start keyword research with a competitor's ASIN or Product ID to search for related and relevant keywords. Cerebro focuses more on the keywords for which a product is ranking near the top in customer search results.