Sales Cloud :- 

“When we develop product in force.com for sales then it comes in Sales Cloud Ex: - Account, Contacts, and Lead”.
In Other Words “ we can say that The Sales Cloud from salesforce.com is the world's #1 sales application. It gives reps, managers, and execs everything they need to connect with customers and focus on what's important —more selling and less administration. It helps you close more deals - faster, gain real-time visibility into sales, and connect with today's social customers”.

Service Cloud :- 
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.
 
In Other words “In service cloud when we sales product to client then we provides the many different types of facilities to client which will be helpful to client just like Call Center.
In call Center ,client can make a call and ask his problem regarding product and can find the better solution about his problem“.

Difference between Sales Cloud and Service Cloud:-
 
1. "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 past, current, and future clients' requests for assistance with a product, service, and billing. You use this to help make people happy.

2. Sales Cloud Implements Sales and Marketing while Service cloud implements Salesforce Knowledge.

3. 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. 

 4. 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.

5. When we develop product 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.

Please check below post for more info
 https://developer.salesforce.com/forums/?id=906F0000000BScJIAW
 
Functions in the Sales Cloud
If you’re currently using or are implementing salesforce.com’s Sales Cloud, you have the foundation necessary to capture and resolve customer questions or issues.  The basic functionality includes:
Cases—you can track customer inquiries, escalate issues requiring specialized support and monitor your agent’s productivity.
Solutions—Equip your customer service team with the answers to your customer’s questions and even make the answers available to the customers on the web so that they can find the answers themselves whenever they need them.
Web-to-Case—if your customers can’t find what they’re looking for on the web, you can have them log a case directly from your website that immediately enters Salesforce.com for agent resolution.

Additional Functionality in the Service Cloud
If you have a more complex support requirements or more active customer service team, you should considerService Cloud licenses.  Service Cloud licenses provides the standard Sales Cloud functionality for your users, and add additional functionality to better equip your team to provide service more efficiently.  You can have Sales Cloud and Service Cloud users in the same instance of Salesforce.com.  Additional features of Service Cloud include:

Agent Console & Customizable Layouts—This helps your agents access relevant information about the customer with fewer clicks.  You can design the console to maximize this efficiency.
Entitlement Management—if you have Service Level Agreements that provide different levels of support for your customers, you can use entitlement management to ensure that you deliver the proper service level including first response and resolution times. You can also incorporate automated processes that provide instructions for reps on the best way to resolve cases.
Access to Live Agent Web Chat (additional fee above Service Cloud license)—Incorporate live chat from the web to ensure that your clients get answers from you in real time without a phone call

http://www.aboveandbeyondcloud.com/comparison-of-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 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 product 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.

Please let us know if this will help u

Thanks
Amit Chaudhary