There are two modules build to respond two different business processes
According to Salesforce
Sales cloud helps customers Grow accounts, find customers, and close deals faster.
Service cloud helps customers empower agents on the path to customer success with smarter, faster service tools.
Comparison table for Sales Cloud and Service Cloud functionality
Difference between Sales Cloud and Service Cloud
Sales Cloud refers to the sales module in salesforce.com. It includes Leads, Accounts, Contacts, Contracts, Opportunities, Products, Price Books, Quotes, and Campaigns (limits apply). It includes features such as Web-to-lead to support online lead capture, with auto-response rules. It is designed to be a start-to-end setup for the entire sales process; you use this to help generate revenue. While “Service Cloud" refers to the "service" (as in "customer service") module in salesforce.com. It includes Accounts, Contacts, Cases, and Solutions. It also encompasses features such as the Public Knowledge Base, Web-to-case, Call Center, and the Self-Service Portal, as well as customer service automation (e.g. escalation rules, assignment rules). It is designed to allow you to support the past, current, and future clients' requests for assistance with a product, service, and billing. You use this to help make people happy.
Sales Cloud Implements Sales and Marketing while Service cloud implements Salesforce Knowledge.
Sales Cloud, A great solution for small and value oriented mid-sized sales groups that want to rapidly and cost effectively deploy Salesforce While Service Cloud provides Customer Support to the Clients and giving you the tools to provide a better customer experience for your clients.
Sales Cloud gives you the ability to open Cases (issues) and relate them to Accounts, Contacts; etc. While The Service Cloud is a superset of Sales Cloud, meaning you get everything that is in Sales Cloud PLUS some other features.
When we develop module in force.com for sales then it comes in Sales Cloud Ex: - Account, Contacts, and Lead. While when we want to provide some facility and also provides support to the clients then it comes in Service Cloud. Ex: - create cases is the example of Service Cloud in which client write his problem into cases instead of call.