Can I export just some of my products instead of the whole catalogue?

Yes, there are two ways, both on Products → Products with the Import / Export plugin active:

• Filter first: apply your filters (for example a category). The Export link under the page title changes to Export (filtered) and downloads only the products that match.
• Pick rows: tick the products you want. A bar appears at the bottom of the screen; click Export there to download only the selected products.

Exports download as a CSV file. The Export link only appears once your store has at least one product.