The two moves
Both grow the value of an existing customer, but in different directions. An upsell is more of the same thing or a better version of it: a higher plan, more seats, a longer contract or a premium tier. A cross-sell is something different that goes with it: a second product, an add-on service or a related item. A software company moving a team from a basic plan to one with advanced reporting is upselling. The same company selling that team a separate training package is cross-selling.
Example
A customer success manager hears that a client's support team now works weekends and keeps hitting its seat limit. She suggests the next plan up, which adds seats and weekend routing: an upsell. A month later, the client mentions their new hires need training, and she introduces the separate onboarding service: a cross-sell.
Why it matters
Selling more to a customer who already trusts you is usually easier than winning a new one, because the relationship, the paperwork and the proof already exist. Growth from existing customers also offsets churn. The risk is that an upsell pitched at the wrong moment, or without a clear reason, feels like the vendor cashing in. Customers accept an expansion when it solves a problem they have already described, not when it arrives because a quota needs it.
How to practice expansion conversations
Good upsell and cross-sell calls look like discovery, not a pitch. Practice asking what has changed since the customer bought, where they're hitting limits and which other teams have similar problems. Then practice connecting one specific option to one specific problem, and handling the customer who says they're happy with what they have, which is a compliment and an objection at the same time.