Hi,

On very high level service Builder is a Liferay tool that generates all the required layers /models to intereact with  database, wheras REST Builder is Liferay tool to build REST and GraphQL APIs. It’s based on OpenAPI, following an API-first approach.

You can refer below for more detailed info  

Rest Builder

https://help.liferay.com/hc/en-us/articles/360028708852-Introduction-to-REST-Builder

https://help.liferay.com/hc/en-us/articles/360028748872-Generating-APIs-with-REST-Builder

https://help.liferay.com/hc/en-us/articles/360036343312-REST-Builder

Service Builder 

https://help.liferay.com/hc/en-us/articles/360017899492-What-is-Service-Builder-

https://help.liferay.com/hc/en-us/articles/360029028211-Introduction-to-Service-Builder

https://help.liferay.com/hc/en-us/articles/360017881932-Introduction-to-Service-Builder