From the publisher's website:
While there are a vast number of spells available in the game’s core rulebooks, many GMs and players have an insatiable desire for more options. In many cases it’s not necessary to create new spell options from scratch, as a few minor changes to an existing spell can create something with a very different feel or impact on game play. These spell variants are based on the idea that there are a lot of interesting effects you can make with fairly minor changes to how a spell works, or by combining effects of multiple spells, or a spell and magic item or even a class ability (preferably from a class not used in the campaign). While we have explored 220 variants in previous products (The Genius Guide to 100 Spell Variants, and The Genius Guide to Another 110 Spell Variants), there’s always room for more magic!
Some spell variants are common and fairly obvious, such as having an iceball spell that is simple fireball dealing cold damage. The following list of 110 spells variants are slightly more complex, and are designed to show how it’s possible to create new spells without too much work. One important tip is that when you base a new spell on an old one many details—such as its school, components, duration, and often even interaction with other spells—is already determined. Similarly, you can often create spells that have entirely new themes just by changing casting time and legal targets. This last trick is often a great way to make spells appropriate for new classes.
For example it’s fairly obvious a druid class shouldn’t have the haste spell on its spell list, but a new spell called pack tactics which acted like haste but could only be cast on creatures of the animal type is perfect for druids. And even better, you already know it’s about a 3rd level spell, since it doesn’t do anything the wizard haste spell doesn’t.