IATS is a payment processing solution used to take electronic donations and payments. It is used for the "Donate Now" button on the website and facilitates payments and donations.
- Relatively good UI
- Good support
- Doesn't have the strongest Salesforce integration
- Better engagement with the Salesforce NPSP
If you need a basic Salesforce integrated payment processor then you could consider using iATS. If you have complex needs and need a complex integration with Salesforce you might want to look elsewhere.
There are some situations where i would choose iATS, but there are other excellent competitors that should be considered as well. Which you choose will depend on your particular needs, the size of your organization and what functionality you need.