What is the difference between up selling and cross selling?

Well, cross selling is to offer the customer an additional product beside the product he decided to buy. if a customer decides to buy a camera for example, the additional product will be a cover for that camera. 

Upselling is to persuade the customer to switch his decision to buy more expensive product of the same type for better quality or more featuers.

Simply put, Up-selling is when you pitch your customer for an upgrade of the same product and Cross-selling is when you pitch your customer with different product - related to first sold product or not - from your product lines.

Eg: If customer has bought32"TV, upselling is upgrading him to54", cross-selling would be pitching the DVD or a speaker system.