What is the difference between the Service Cloud and Sales cloud in Salesforce?

Sales Cloud give you Web-to-lead functionality, whereas Service Cloud gives you Web-to-case functionality. Salesforce Service Cloud makes connecting one-to-one with every customer, across multiple channels on any device, extremely easy.

Sales Cloud is about core CRM and the Sales process. So it will hold all your account data with relevant contacts and linked with it opportunities and their related objects like contracts.
In Service Cloud you mainly talk about cases, which in itself need to be linked to accounts & contacts. But the focus on service cloud is case management, so think call centres as well as service engineers like you have for utilities.