> For the complete documentation index, see [llms.txt](https://glu.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://glu.gitbook.io/docs/concepts.md).

# Concepts

- [Contacts](https://glu.gitbook.io/docs/concepts/contacts.md): Learn what a contact represents and how best to handle them/
- [Custom fields](https://glu.gitbook.io/docs/concepts/contacts/custom-fields.md)
- [Memberships & Loyalty](https://glu.gitbook.io/docs/concepts/memberships-and-loyalty.md): Learn how to craft a membership program
- [Products](https://glu.gitbook.io/docs/concepts/products.md)
- [Gift cards](https://glu.gitbook.io/docs/concepts/products/gift-cards.md)
- [Physical goods](https://glu.gitbook.io/docs/concepts/products/physical-goods.md)
- [Memberships](https://glu.gitbook.io/docs/concepts/products/memberships.md)
- [Creating orders](https://glu.gitbook.io/docs/concepts/products/creating-orders.md)
- [Multi-currency](https://glu.gitbook.io/docs/concepts/products/multi-currency.md): Understand how to handle selling across multiple currencies.