In marketing sense,

Upsell refers to selling a similar and expensive product/service.
For example: You walk in to a mobile phone shop wanting to buy a basic phone. However, the salesman is trying to sell you the latest model of Apple iPhone. In this context, we say that the salesman is involved in upselling because an iPhone is a phone and it is more expensive than a mobile phone.

Cross-sell refers to selling a completely different product/service.
For example: You walk in to a mobile phone shop wanting to buy a basic phone. However, the salesman suggests that you should upgrade your mobile plan to enjoy free data access. In this context, the salesman is involved in cross-selling because a mobile plan is completely different from a mobile phone.