The main difference is that in Micro and Basic you can only check for HTML issues, while in Pro, Agency and Enterprise you can also scan for accessibility issues, and have access to advanced features like scheduling, muting and device viewport emulation. Micro has the same features as Basic, but less capacity and speed.
Agency plans are designed for web agencies and companies, providing increased capacity and performance. An Agency plan provides 2x times more credits than a Pro plan, and admits up to 10 guest accounts so you can have your development team working together in your site validations.
Enterprise plans are designed for large teams and organizations, providing increased capacity and performance. An Enterprise plan provides 10x times more credits than a Pro plan, and has higher speed and report storage capacity.